List window previews horizontally if panel is horizontal

Currently if window previews are enabled they're always listed vertically. This looks strange when the panel is horizontal (see screenshot below) as most other OSes and desktops with taskbar previews display them horizontally in this case.

This MR automatically changes the orientation of the Group Menu to horizontal if the panel is horizontal and window previews are enabled, like so:

If the panel is vertical, the window previews stay vertical like before, as this makes more sense:

If window previews are disabled, the Group Menu stays vertical like before even if the panel is horizontal, as this is more space-efficient: