cursor: add cursor_update_focus()

...and call it from desktop_move_to_front() in order force an enter event
on the surface below the cursor when cycling views.

Inspired by PR #164 - just restructured it a bit.

Suggested-by: @bi4k8

src/desktop.c

@@ -61,6 +61,7 @@ desktop_move_to_front(struct view *view)
 #if HAVE_XWAYLAND
 	move_xwayland_sub_views_to_front(view);
 #endif
+	cursor_update_focus(view->server);
 }
 
 static void