Drop support for Glib < 2.36
Drop deprecated g_type_init, type initialized automatically.
Drop support for Gtk+ < 2.24
Cleanup GTK_CHECK_VERSION and unify gtk+2/3 code where possible

gdk_x11_set_sm_client_id | Since 2.24
gdk_x11_display_get_xdisplay | Since 2.2
GDK_DISPLAY_XDISPLAY | Since always
GDK_WINDOW_XID | Since always
gdk_window_get_screen | Since 2.24
gdk_x11_window_foreign_new_for_display | Since 2.24
gdk_window_get_width | Since 2.24
gdk_window_get_height | Since 2.24
Remove deprecated gtk_dialog_set_has_separator
GDK_KEY_* was introduced in Gtk+ 2.22
Replace deprecated GTK_WIDGET_SET_FLAGS