Improve Cancel behavior when loading a remote database

[keepassium]

When loading a remote database on a poor connection, it takes a while for the process to finish or fail. Having an option to abort the remote loading attempt and switch to the offline copy would save some users quite a bit of waiting.

So, when loading a remote database that also has an offline copy, the "Cancel" button in DB Unlocker's progress should show a menu:

• Stop (the original "Cancel" action)
• Load Local Copy (that is, consider remote file unresponsive and fall back to the offline file).

[Thanks, @jasperweiss #391 and O.S.S.]