Add graphics capabilities to handle new features in Inspector.py

[lisham2000]

No description provided.

[cmeyer]

I think it may make things easier to make the following changes:

• assume line width is always a positive float, >= 0.
• use the None-ness of fill_style and stroke_style to determine whether they are enabled

Doing these two changes will eliminate the redundant new persistent fields is_stroke_enabled and is_fill_enabled, although you might still want to implement them as observable properties dependent on stroke_style and fill_style. This has the downside that disabling and then re-enabling them will lose their particular value; but it will simplify the code for now. We can address the disable/enable workflow in the future.

[lisham2000]

@cmeyer Agree in hindsight keeping the float value as a float and not an optional means less downstream changes and means no PR is needed for nionUI. This downstream problem was only discovered when testing backwards compatibility with Kev when it had already been implemented and we discussed whether it was sensible to change and were on the fence about it so if you think going for a simpler solution is better for now will update and put through.