clients/window: Add functions to set/retrieve app_id

Adds appid for all clients using the toolkit, flower, fullscreen, image,
resizor, scaler, smoke, stacking, subsurfaces, terminal,
touch-calibrator, transformed, etc.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
diff --git a/clients/scaler.c b/clients/scaler.c
index 91736fb..ed9ae62 100644
--- a/clients/scaler.c
+++ b/clients/scaler.c
@@ -303,6 +303,7 @@
 	box.window = window_create(d);
 	box.widget = window_add_widget(box.window, &box);
 	window_set_title(box.window, "Scaler Test Box");
+	window_set_appid(box.window, "org.freedesktop.weston.scaler-test-box");
 	window_set_buffer_scale(box.window, BUFFER_SCALE);
 
 	widget_set_resize_handler(box.widget, resize_handler);