Drop support for glib < 2.36
Replace deprecated g_format_size_for_display with g_format_size
Use g_spawn_async for gtk+-2 and gtk+-3
Cleanup GTK_CHECK_VERSION and unify where possible

gdk_display_get_default | since 2.2
gdk_x11_display_get_xdisplay | since 2.2
gdk_drag_context_list_targets | since 2.22
gdk_x11_window_foreign_new_for_display | since 2.24
gdk_drawable_get_screen | since 2.24
gtk_widget_hide | always
Replace GtkObject with GObject also for gtk2
Remove deprecated call to g_thread_init