Cleanup GTK_CHECK_VERSION and unify where possible

gdk_x11_window_lookup_for_display since 2.24
GDK_WINDOW_XID since always
gtk_widget_get_visible since 2.18
gtk_widget_set_mapped since 2.20
gdk_event_new since 2.2
gdk_x11_window_lookup_for_display since 2.24
gdk_text_property_to_utf8_list_for_display since 2.2
gtk_widget_get_realized since 2.20
gdk_visual_get_depth since 2.22
gtk_widget_get_window since 2.14
gtk_widget_set_allocation since 2.18
Drop support for Gtk+ < 2.24
Drop support for Glib < 2.32