Support for stopping motion/blanking display outside of trial/during feedback

[bboudaoud-nv]

UL has requested that we add support for disabling player motion when instructions are displayed prior to a trial.

Similarly we've had requests to blank all rendered output outside of trials to help reduce perception of stutters when we go to disk outside of the trial time.

We could potentially combine these two requests and add support for a feature that:

1. Allows the experiment designer to disable player motion prior to the trial (could default to enabled)
2. Allows the experiment designer to specify some static 2D content/color to display (for each state?) outside of the trial
3. Allows for a mix of (1) and (2) above in each of the possible session states

[bboudaoud-nv]

We should check and see if disablePlayerMotionBetweenTrials covers this request and if so close the issue. My guess is that this may be referring to not having the view rotate between trials as well. I think the PlayerEntity::m_motionEnable just disables calling the PlayerEntity::slideMove() method so view rotation would still happen here.