Rename Properties to Preferences, move config migration to Preferences
Refactor Preferences
Add Application class, handle global shortcuts there
Use MainWindow instead of QMainWindow in Application
Simplify accessing Preferences in PropertiesDialog
Rename PropertiesDialog to PreferencesDialog

And some additional renamings of Properties to Preferences.
Use "GNOME" as XDG DE on LXQt

This makes Qt to load correct icon theme