plugins/overview: Make screen edge toggle overview rather than cycle between modes
The user may have no usecase for the grid view mode, i.e. they may want to activate overview, do their thing, and then return back to normal desktop. However, with the current behavior, there's one extra step (switch to grid mode) in order to go back to desktop.
In hindsight, we should have added different screen edge actions for overview and grid modes. This can be done in 6.1.