"Next Slot" control mapping

-added "Next Slot" to control mapping menu
-added VIRTKEY_NEXT_SLOT -> incremented VIRTKEY_TOGGLE_FULLSCREEN
-added SaveState::NextSlot(); as replacement for code in
WndMainWindow
-added entry in EmuScreen using SaveState::NextSlot();
When loading a savestate with more ram, realloc.

Also, I'm not sure the extra ram is being handled correctly, I think it's
not meant to be exposed to games except homebrew, not sure of the exact
method...
Revert the gp struct member in mpegs for now.

Unlikely to be used, even though seen in tests, and could be
causing #5045 (just a guess.)