keyboard: add keyboard_get_all_modifiers()

keyboard: add keyboard_get_all_modifiers()

And make mousebind handlers use that one.
Also remove keyboard_any_modifiers_pressed() and replace its usage
with the new function.

Without this patch we would only request the modifier state of the
keyboard group which makes mousebinds involving keyboard modifiers
break for virtual keyboards like when using wayvnc. Same story for
hiding the workspace overlay or snapping to regions.

Fixes: #2511