Fix for #547. You can now play a sound from any frame in its duration, not just its starting frame.

You can also delete and move the sound from any frame.

Code Changes:

• Added "mCheckForSoundsHalfway" flag to PlaybackManager. Set to true when starting playback - to check whether to play any sound from part-way through.
• Added "getKeyFrameWhichCovers(pos)" to Layer. This returns the keyframe which covers pos. E.g. a sound clip starting at frame 1 and ending at frame 6 covers frames 1, 2, 3, 4, 5, 6.
• Updated Layer methods "isFrameSelected(pos)" and "setFrameSelected(pos)" to use "getKeyFrameWhichCovers". I could possibly move this logic change to the LayerSound class and keep Layer as is.
• Added "playFromPosition()" to SoundClip to play sound from part-way through.