Revert "License no longer installed on user's machine."

Installing LICENSE file on user's machine after all (even though the
license-related link in AboutDialog now links to license on Apache website).

This reverts commit ae5eaee15ad200379cc6f16bb9ff6ac48fde1be7.
Deleted obsolete function App::license_url().
Various improvements to AboutDialog and related code.
Deleted an obsolete TODO.