Restore support for dact: filter URIs.

The support for URIs was disabled after the move to multi-window support. It
is now added back with the following heuristics:

- If there is at least one open window, apply the filter in the active
  window.
- Otherwise, prompt the user to open a corpus and apply the filter to that
  corpus.
Merge branch 'master' of github.com:rug-compling/dact