Play/Pause + interactive slider bar

_pausedFrame was added to the Player class but is not initialized in the ctor.  Since _pausedFrame is only ever set when _isPaused is set true then we can conclude two things:  (1) in theory _pausedFrame doesn't actually need to be initialized in the ctor (whereas in practice it is probably a good idea) and (2) _isPaused and _pausedFrame could be combined into a single variable (_pausedFrame) for which an invalid value (such as -1) could indicate !_isPaused.

This doesn't block the merge.  I merely point it out.  Nevertheless, would expect the two variables to eventually be combined in some future overhaul of the code -- I've done similar cleanups on my own and other's code over the years.