As described in #24248, the workaround employed by
`GlobalActionContainer`, wherein it tried to handle actions with
priority before its children by being placed in front of the children
and not _actually containing_ said children, is blocking the resolution
of some rather major input handling issues that allow key releases to be
received by deparented drawables.
To resolve, migrate `GlobalActionContainer` to use `Prioritised`, which
can be done without regressing certain mouse button flows after
ppy/osu-framework#5966.
- Only displays when required (there's literally zero case we want to
return input to the test, as this is automatic on next action)
- No longer hugs the right side of the screen (blocking visibility of
some tests).