appimagetool uses the wrong icon

Continuation from https://github.com/probonopd/linuxdeployqt/issues/126

The desktop file specifies `Icon=Nextcloud` but then linuxdeployqt decides `iconToBeUsed: "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_error_shared.png"`. This is a bug, probably caused in https://github.com/probonopd/AppImageKit/blob/adaf3c5a2576da78d7268836c02f04c0106149d9/shared.c#L135-L192

PRs welcome!

```
root@a5a478a42159:~# ./squashfs-root/AppRun /app/
AppRun etc/ usr/
root@a5a478a42159:~# ./squashfs-root/AppRun /app/usr/share/applications/nextcloud.desktop
Desktop file as first argument: "/app/usr/share/applications/nextcloud.desktop"
desktopExecEntry: "nextcloud"
desktopIconEntry: "Nextcloud"
Found binary from desktop file: "/app/usr/bin/nextcloud"
FHS-like mode with PREFIX, fhsPrefix: "/app/usr"
app-binary: "/app/usr/bin/nextcloud"
appDirPath: "/app"
relativeBinPath: "usr/bin/nextcloud"
Copied "/app/usr/share/applications/nextcloud.desktop" to "/app/nextcloud.desktop"
Found icons from desktop file: ("/app/usr/share/icons/hicolor/22x22/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/512x512/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_sync_shared.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_sync.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_ok.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_warn_shared.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_ok_shared.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_error.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_warn.png", "/app/usr/share/icons/hicolor/32x32/apps/Nextcloud_error_shared.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_sync_shared.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_sync.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_ok.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_warn_shared.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_ok_shared.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_error.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_warn.png", "/app/usr/share/icons/hicolor/48x48/apps/Nextcloud_error_shared.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_sync_shared.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_sync.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_ok.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_warn_shared.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_ok_shared.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_error.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_warn.png", "/app/usr/share/icons/hicolor/64x64/apps/Nextcloud_error_shared.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_sync_shared.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_sync.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_ok.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_warn_shared.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_ok_shared.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_error.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_warn.png", "/app/usr/share/icons/hicolor/128x128/apps/Nextcloud_error_shared.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_sync_shared.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_sync.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_ok.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_warn_shared.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_ok_shared.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_error.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_warn.png", "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_error_shared.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_sync_shared.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_sync.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_ok.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_warn_shared.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_ok_shared.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_error.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_warn.png", "/app/usr/share/icons/hicolor/16x16/apps/Nextcloud_error_shared.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_sync_shared.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_sync.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_ok.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_warn_shared.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_ok_shared.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_error.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_warn.png", "/app/usr/share/icons/hicolor/72x72/apps/Nextcloud_error_shared.png")
iconToBeUsed: "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_error_shared.png"
Copied "/app/usr/share/icons/hicolor/256x256/apps/Nextcloud_error_shared.png" to "/app/Nextcloud_error_shared.png"
```