OpenDingux: Add a hotkey to toggle fast-forward until it's next held.

It can work in tandem with the fast-forward while held hotkey. The rationale, as written by gamefreak_joey in <http://boards.dingoonity.org/gcw-releases/regba-gba-emulator-%28alpha-1%29/msg83617/#msg83617>, is for extended grinding sessions in RPGs such as Pokémon. The hotkey which fast-forwards while held would get tiring to keep holding.