Fill color

[edemaine]

I think outline color should be the primary interface, but for rectangles, ellipses, and polylines, it'd be nice to set fill color separately from stroke color. Then you can make a nice outlined box. Also ideally fill opacity (and someday stroke opacity?).

Proposed interface for fill color:

• Next to color palette, there's a button/toggle for turning fill on or off (default off). It also shows the current fill color. I'm imagining a bucket icon on top of the fill color, with a slash through it when fill is off. (Also the button can be up or down.)
• Shift clicking or right clicking a color sets the fill color to that color. (Regular clicking still sets stroke color.)
• Maybe also dragging color to fill button?

(from #35)