Drawing without stroke

[edemaine]

When drawing filled pixels, it's annoying to have to change both stroke and fill color every time. One convenient way would be to turn off the stroke. Possible interfaces:

• Line width 0 is selectable (but then meaningless without fill)
  • I've experimented with this slightly. It makes lines impossible to select via direct clicking, even if we boost 0 to say 0.01. Also, the Collision algorithm doesn't work anymore, so can't select in the middle of a line using a rectangle; it seems to assume nonzero width.
• Transparent color option (could be used to turn off stroke or fill).
  • Most drawing programs go this way.
• Stroke toggle (but then meaningless without fill)
• 3-way toggle between stroke, stroke+fill, and fill only replacing the current fill toggle (drop):
  • 
  • This is nice because it slightly more consistently shows your stroke and fill color. (Currently, fill is in the drop and stroke is in the line widths and color selector.) But the behavior is maybe confusing...

Related, we need to figure out how the pen tool interacts with this. Presumably don't want pen at zero width, unless we also allow pen to fill? (Currently we don't.)