"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();
Merge pull request #5040 from zack-vii/NextSlotKey

"Next Slot" control mapping