Possible approach to disposing of SWT text block resources

This is an experiment (since I haven't been able to test on Windows while traveling :airplane:) in getting `TextBlock` related SWT resources disposed of properly.

The main idea here is to make `FittedTextLayout` the central player in controlling the resource lifetimes. It already holds the layout we need to get rid of, it there when we're creating fonts and colors, and (can) get directly called when disposing/relaying things out.

Because of timing of when I was working on this, it overlaps some with what @wasnotrice was doing with #655, but I view this as the next step (since it does layouts as well as fonts and colors). Would be perfectly happy if we want to take #655 and I can this before we land it.

Also keen to discuss this too since getting a good pattern of how we handle the resources will be worth a bit of extra thought. Fire away with anything you're thinking about here!