Add a way to get the current page in a menu

[geominorai]

There is currently no reliable way to get the current page a client is viewing for a Menu outside of Menu.Selection that only works in the MenuAction_Select callback.

For some applications, we would like to know the page or first item on the page like Menu.Selection provided when the client is simply flipping through menu pages with Next and Back. For example, to refresh values on a menu and seamlessly show it back to the client viewing it by using Menu.DisplayAt to jump back to the same page.

[asherkin]

This looks like it'll just be a case of exposing player->states.item_on_page, and it looks like the best way to expose that will be to extend GetClientMenu (see also #1552).