Expose the region used for a temporary popup to the context so it's
possible for scripts to access the popup-region they're being drawn in.
Note that the internal naming is currently "menu" which is misleading
as these are used for all kinds of dialogs & popups. The RNA member is
called `region_popup` in this commit,
we could consider changing the name internally too.
Needed so it's possible for scripts to access the splash screen
so it can be refreshed, see: #120612.
742a8b47cd