Minor UI issues

[CGrassin]

There are a few minor issues in the UI of the plugin. They don't affect usability at all, just look and feel!

1. Track clearance is the only action without a category. It should likely be "Modify PCB".
2. Measure length is the wrong category. It should likely be "Read PCB".
3. Solder mask expansion renders the dialog in a strange way, with plenty of whitespace:
   
4. Taper outputs some debug information, I don't think that should be shown (add a debug flag in the code?)
   
5. Track clearance has not title, and also renders with plenty of whitespace
   

I intend to make a PR to fix those.

[easyw]

I intend to make a PR to fix those.

@CGrassin
thanks for pointing this out.
Please check also the behavior in high resolution screen with scaling DPI setting