For the hot corner case, we want to have the pressure apply both in
and outside of the overview, so we need to move this to the user. At
the same time, use keybinding mode math that's more like what's used
in filterKeybinding.
While it may seem like an abuse of the KeyBindingMode API, it may
become more reasonable if one thinks of the pressure barrier as a
binding of sorts, just applied to the mouse. If a ButtonBinding API
was added to mutter, I think we'd use the existing KeyBindingMode
infastructure there as well.
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