Implement a uses-widget that is actually useful.
It groups the uses by file, then by function or class-name, and shows the source-complete surrounding source-line with the range highlighted for each use.

The tooltip shows even more context.

Yet todo:
- Make pure keyboard-navigation work
- Make all the buttons much smaller, somehow the oxygen buttons are really huge
- Update duchain to get _all_ and up-to-date uses.