Menu navigation enhancements

[InfiniteBlueGX]

These changes improve menu navigation within the GX GUI:

• The user can now press the B or 1 button to quickly return to the previous menu, for all menus except those with more than one button outside of an OptionBrowser (controller button mappings menu for example - you need to focus out of the OptionBrowser to select "Reset").
• The in-game menu can now be closed with the GC controller's Start button, in all the same places where closing with the Home button is allowed (any GUI screen that has a "Close" button).
• All existing functionality of UI elements should be preserved.

I recommend trying these changes out to get a feel for them. Personally, I find them to greatly enhance menu navigation when not using the Wiimote IR pointer. If merged, I will definitely port over these changes to FCEUGX in the future.

[saulfabregwiivc]

This is fine, but not just FCEUGX and Snes9xGX needs this, don't forget also about VBAGX! :D

[dborth]

Looks reasonable to me, thanks for the PR!

[InfiniteBlueGX]

This is fine, but not just FCEUGX and Snes9xGX needs this, don't forget also about VBAGX! :D

Due to how mGBA Wii completely outclasses VBAGX in virtually every way regarding emulator accuracy and performance, I personally don't see value in spending time to support VBAGX going forward (no offense).

If someone else wants to port my contributions over themselves, they're more than welcome to!