move pause out of gbs code?

[mmitch]

Should pause handling be delegated to the player code and be removed from gbs.c and gbhw.c?

If a player plays multiple GBS files in parallel (which will be possible with issue #41) and pauses one of then, then the paused gbs core will call nanosleep() when paused. This will affect the other GBS files as well.

Is there any need for the pause code to be in the core?
The player code can just stop calling gbs_step() and the core is effectively paused – no code needed in the core for that.

[mmitch]

I did this – it's merged to master.