Add Gear::onStartPlaying and Gear::onStopPlaying callbacks

[sofian]

I'm running into the problem at this point where movies from a file are read in a separate thread (GStreamer) so when I start/stop Drone it doesn't start/stop the movie. Whenever the engine calls startPlaying() and stopPlaying() it should correspondingly call onStartPlaying() and onStopPlaying() in gears.

Gear::prePlay and Gear::postPlay are also confusing (see how they are used in Engine.cpp) and should probably be renamed.