Debug feature: Frame advancing

[Ghabry]

Suggested by IndependentArt:
It should be possible to slowdown the Player.

Imo more useful is a frame advance feature. Frame advance means that you can progress frame by frame.

Possible implementation to not require many more buttons:

Add a new key (P?) for PAUSE which suspends the execution and while Pause is active the fast forward key F will Frame advance instead.

As with fast forward this will not alter the audio playback because our API does not support pausing (this is semi-problematic because event code can probe the audio playback position, luckily not many games use this).

Just as a related note. It seems a baseline Steam requirent is to provide a game pause feature, so having it beforehand to enable this would be useful for both.

Asked via email for a "slow button mode", tightly related to this. Assuming this could be a x0.5 or x0.1 modes, equivalent to fast forward mode. There were no details in the email report, but the frame by frame looks indeed useful for debugging, too.

For a simple key solution for both, holding the F for more than 1 second while paused could toggle some x0.5 forward, or use G for switching between different speed modes without holding or something like that.