osd: prevent preview outlines to be above OSD

osd: prevent preview outlines to be above OSD

86c3597 raised server->menu_tree above layer shell surfaces, but it caused
preview outlines for window switcher to be placed above the OSD because
the preview outlines are placed just above server->menu_tree.

So this commit fixes it by placing server->osd_tree above server->menu.
This also prevents layer-shell surfaces from covering the OSD.

Before (higher-first):
- preview outlines (just above menu)
- menu
- layer-shell (top/overlay/popup)
- osd

After (higher-first):
- osd
- preview outlines (just above menu)
- menu
- layer-shell (top/overlay/popup)