Fix PgUp/PgDown handling in overview
The keys are supposed to be usable for switching between workspace and app pages, but both are broken right now:
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which ones gets to handle the event is based on the mapped state, but nowadays both are always mapped (workspaces win)
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the handling for workspaces still tries to switch workspaces vertically, which obviously doesn't do anything anymore
This fixes both, but in a slightly questionable way as it relies on workspaces getting first dips (the alternative is to give the app grid access to the overview state, which is a bit more intrusive).