Cleanup PluginLoader
Add plugin stub for Windows
Add DockPlugin
Move dock alert to Windows & OSX plugins
Hide system tray on Gnome
Gnome: implement WindowPlugin interface
Gnome: X11 helper

The whole Gnome integration is now in a separate plugin. It
can depend on X11 (extras) for now, until Wayland support is
implemented sometime in the future.
Gnome application menu integration by Venemo