Tweaks to Layer

Layer::go() will now go to the first cue in the list if the current cue
is negative (invalid)
CueList::getPrevCue() had a bug.
Loading playback data shouldn't reset current cue

Added more cues to test files
Bug fixes for Cue

-Cue loading from JSON wouldn't initialize length or type status
-Additional functions for getting first and last keyframe data
Merge remote-tracking branch 'upstream/cues'