[draft] Let user specify a fixed size or aspect ratio for rectangles/ellipses

[mvandenburgh]

Closes #26

[mvandenburgh]

@manthey this isn't functional yet, but is this UI in line with what you were thinking?

When the user selects rectangle or ellipse, an additional dropdown will appear under the existing style group dropdown, which allows them to choose between unconstrained, fixed aspect ratio, and fixed size. If they select unconstrained, nothing happens. If they select one of the other options, a popup dialog appears so they can specify a height and width.

[manthey]

I was imagining a very compact control like:

Where clicking the arrow would drop down out a small set on controls with the options (rather than open a dialog). My thought was that by having it more compact, it would feel quicker to change.

[mvandenburgh]

Closing in favor of #197