Add dropdown to enable drawing shapes with fixed aspect ratios/sizes

[mvandenburgh]

Closes #26

[manthey]

We weren't passing the mode options through. I've made a PR in large_image to do this: girder/large_image#881
You'll need to change the code to something like:

const options = {modeOptions: {}};
options.modeOptions.constraint = { width: this._editOptions.fixed_width, height: this._editOptions.fixed_height };

[manthey]

I noticed that the dropdown arrows on the control panels were inconsistent because of how I had implemented them. I pushed a change to fix that.

Otherwise, this is working for me. I'll approve this -- if you don't like my changes, feel free to un-approve and handle things a different way.

[manthey]

I think you'll be able to merge without the coverage test passing.