Set _NET_WM_ICON so that Dock shows correct icon

Currently, the dock only shows the correct icon if an AppImage is either launched through a desktop file and/or the application explicitly sets a window icon (e.g., using Qt) like this:

```
QFileInfo fi(app.applicationDirPath() + "/../share/pixmaps/qtquickapp.png");
QGuiApplication::setWindowIcon(QIcon(fi.absoluteFilePath()));
```
which seems to set the `_NET_WM_ICON` according to https://git.qt.io/consulting-usa/qtbase-xcb-rendering/commit/6131b909532f7bfdcbc8fa2f35f2c6f84c06547d.

Could we make the AppImage `AppRun.c` read the icon from the desktop file, and have it set the application's `_NET_WM_ICON` programmatically?

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=465109 may contain some source code that might do it.