Fix deprecation warnings in several modules

This is completely untested, and only fixes the low-hanging fruit.

Remaining issues:

1. libscreenshot uses gdk_screen_get_active_window, which according to the deprecation warning, doesn't have a substitute.
2. libnotifications uses the gdk_screen_get_width and _height. Is it OK to use monitor geometry instead?
3. libnotifications uses the integer result of gdk_screen_get_monitor_at_point to directly access a local array. The code has to be rewritten as the replacement function returns an object. It can probably be replaced by a hashmap on the monitor pointer.