Enhancement : Shortcut to toggle onion skin / layer preview

[juliandescottes]

Three different states for the overlay displayed on the drawing area :

• onion skin (display previous and next frame)
• layer preview (display merged bottom layers and merged upper layers)
• nothing

Provide keyboard shortcuts and accessible buttons to enable/disable these overlays without having to open the user preferences drawer.

[juliandescottes]

Work in progress can be seen at http://juliandescottes.github.io/piskel/

Two new icons :

• Animated Preview Panel : toggle onion skin
• Layers Toolbox : toggle layer preview

Added 2 keyboard shortcuts for toggling onion skin / layer preview :

• alt L : toggle layer preview
• alt O : toggle onion skin

Will keep playing with this over the week end, before redeploying to piskelapp.com

[juliandescottes]

Merged and deployed