window: Make decorations just a widget

Woohoo, only took 25 commits of refactoring to get to this point.
diff --git a/clients/window.h b/clients/window.h
index 3252c89..051c690 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -281,9 +281,6 @@
 window_get_user_data(struct window *window);
 
 void
-window_set_decoration(struct window *window, int decoration);
-
-void
 window_set_key_handler(struct window *window,
 		       window_key_handler_t handler);
 
@@ -349,6 +346,9 @@
 void
 widget_schedule_redraw(struct widget *widget);
 
+struct widget *
+frame_create(struct window *window, void *data);
+
 void
 input_set_pointer_image(struct input *input, uint32_t time, int pointer);