Onion skinning

[charlielee]

Reimplement onion skinning with some additional options.

• Switch to toggle showing the last frame and the "pinned" frame
• To pin a frame: select it in the frame reel and choose the pin button
• Option in the sidebar to set the number of onion skin frames to show
• Center of the slider should be 50% rather than doing the same thing if it is moved left or right
• Default onion skinning percentage and number of frames should be stored in preferences