Various issues regarding DesktopEntryProvider and SystemAppProvider (Linux)

[Cybso]

1. The applications directories should be search recursively
2. Look for desktop entries in QStandardPaths.AppConfigLocation instead of QStandardPaths.AppDataLocation, since they are part of the configuration
3. Execute desktop files using Shell (or try http://pyqt.sourceforge.net/Docs/PyQt5/api/qdesktopservices.html ). Example of a desktop file that currently does not execute:

[Desktop Entry]
Name=The Secret of Monkey Island - Special Edition
Exec=env WINEPREFIX="/home/steam/.wine" wine-stable C:\\\\windows\\\\command\\\\start.exe /Unix /home/steam/.wine/dosdevices/c:/users/Public/Start\\ Menu/Programs/GOG.com/The\\ Secret\\ of\\ Monkey\\ Island\\ -\\ Special\\ Edition/The\\ Secret\\ of\\ Monkey\\ Island\\ -\\ Special\\ Edition.lnk
Type=Application
StartupNotify=true
Path=/home/steam/.wine/dosdevices/c:/GOG Games/Monkey Island 1 SE/
Icon=3748_goggame-1207666253.0
Categories=Game

4. Include desktop in SystemAppProvider under Linux