Add Wayland support
- Use gtk-layer-shell to place notification windows in the top layer. We also have to use layer-shell's anchor and margin settings in order to place the windows (
gtk_window_move()
doesn't work on Wayland here). The margin settings are also used to move the window for the slideout, when enabled. - Bump GTK requirement to 3.22. This gives us all the
GdkMonitor
stuff, andgdk_monitor_get_workarea()
. 3.22 was released in 2016, so I think this is long overdue, and it lets me delete a lot of X11-specific code as well as some GTK-version-checked helpers. - Surround the remaining X11-related stuff with
#ifdef
s.