Re-add command in links

[PoignardAzur]

When porting from Druid to Masonry, I've removed the ability to trigger commands from links.

While I'm not sure how to handle that feature on the long term, on the short-term it should be fairly easy to re-enable. Most of the code is still there, just commented out.

Essentially, we just want Label widgets to send Commands during MouseUp events when the cursor is over a link. Ideally we'd want to handle active status and cursor changes and stuff, but we'll keep it simple for now.