AppRun binary does not comply with FreeDesktop Desktop Entry specification

Separating the AppRun issue from #678.

After even further testing, there IS a problem with AppRun. I did some reading to brush up on the spec for .desktop files. There is a way to escape a space, and other characters. According to the spec and my testing, `Exec=Katawa\\ Shoujo.sh` should work. Indeed, if I point the Exec line to the script using the existing environment like `Exec=~/AppDirs/Katawa\\ Shoujo-x86_64.AppDir/usr/bin/Katawa\\ Shoujo.sh`, and double-click the .desktop file, it works.

However, `Exec=Katawa\\ Shoujo.sh` and running via AppRun does not work: `Error executing 'Katawa\\': No such file or directory`. AppRun is therefore a non-compliant implementation of the [FreeDesktop Desktop Entry specification](https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#exec-variables)