shell: Allow unresponsive surfaces to be moved again

Commit c85f1d45 caused the move of an unresponsive surface to be no
longer possible, since the grabbed flag would prevent the move grab
to start while the busy grab was still active.
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index 466ea93..ba5f2f3 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -1807,6 +1807,9 @@
 
 	shell_grab_start(grab, &busy_cursor_grab_interface, shsurf, pointer,
 			 DESKTOP_SHELL_CURSOR_BUSY);
+	/* Mark the shsurf as ungrabbed so that button binding is able
+	 * to move it. */
+	shsurf->grabbed = 0;
 }
 
 static void