Enable onion skinning from a frame other than last frame

[charlielee]

• Dragging the onion skin slider to the left should allow you to use a frame other than the last one
• Add a button to the animation toolbar to mark a frame as the "onion skin frame" (need a better name for this)
• Icons and tooltips should be added to the left and right of the onion skin slider to try and make it clearer how they act differently.
• The status bar should state the selected onion skin frame number

From #71:

Drag to the left: Onion skins last frame taken with live preview if on live mode. If you select another frame in the reel, it'll use that for onion skinning instead.