Fixed recently introduced bugs in mergeLayer, cloneLayer.
More Draw unit tests.
Migrating Canvas methods to Draw, Layer, and utilities: copyElem,
pathActions.convertPath, cloneLayer, mergeLayer, and mergeAllLayers.
Canvas API is unchanged for backwards compatibility (i.e. previous
functions forward to new functions).
Canvas was referencing drawing.all_layers and drawing.current_layer.
Both variables now represent Layer instead of group element and should
be considered private.
Moved Layer class to layer.js
New HistoryRecordingService added to help with moving Layer code out of
Canvas. Started using it in Canvas.mergLayer