Make side bearings editable interactively by dragging them

[justvanrossum]

The basic part of this isn't very difficult.

The scene + canvas layout is affected by advances, and this behavior needs to be more controlled for dragging left and right sidebearings work well.

[justvanrossum]

Some notes:

• Add sidebearing/left and sidebearing/right to possible selection items (this may interfere with parseSelection(), perhaps it should be sidebearing/0 and sidebearing/1, to be continued for vertical?)
• Make sidebearings hoverable and selectable
• Make sidebearings draggable (dragging the left one should make the canvas scroll in the opposite direction)