shell: Don't move maximized window

We'll want to ask the client to unmaximize once we get support for that in
xdg-shell, but for now, just refuse moving a maximized window.
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index 2b9241c..2f8e610 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -1475,7 +1475,7 @@
 
 	if (shsurf->grabbed)
 		return 0;
-	if (shsurf->state.fullscreen)
+	if (shsurf->state.fullscreen || shsurf->state.maximized)
 		return 0;
 
 	move = malloc(sizeof *move);