window: Compute initial window size correctly

We didn't take decoration size into account before.
diff --git a/clients/window.h b/clients/window.h
index da3815b..92e52a9 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -363,6 +363,8 @@
 
 struct widget *
 frame_create(struct window *window, void *data);
+void
+frame_set_child_size(struct widget *widget, int child_width, int child_height);
 
 void
 input_set_pointer_image(struct input *input, int pointer);