Add stroke properties for line corner styling

[Keavon]

https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke-linecap
https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke-linejoin
https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/stroke-miterlimit

Ask me to make some icons for these and I can whip them up to use with the radio button input, or just use text for now.

These should be part of the stroke section of the Properties panel.

Similar task: #578