Fix : bugs in imageToBlob and PngExportController
Feature : add onion skin option

New option in application settings : onion skin.
You can choose the overlay to display now :
- no overlay
- onion skin (default)
- layer preview (previous default)

Available in Application Settings panel.

Only one overlay can be used at the same time.
The onion skin overlay is driven by a new OnionSkinRenderer maanged by the
drawing.

The drawing controller is responsible for instanciating and 'choosing' the
overlay renderer.

When switching to a new overlay, other overlays are cleared and flushed
(they cache their rendering frame, flush empties the cache).

NB : flush is only available on LayersRenderer and OnionSkinRenderer for
now.