On request of GNOME Classic users, we add GNOME2-like workspace previews
when using a horizontal workspace layout. The previews scale a lot worse
than the menu though, with the risk that they take up all the available
width in extreme cases.

Address this by also taking the number of workspaces into account, and
switch to the menu when we have more than six.

This is particularly important now that we switched to a horizontal
layout by default.

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