Add "check for updates" option

[Popolechien]

We do have users that are occasionally online, and it would be nice to add a "check and install update" option so they don't need to come back to kiwix.org just to look up the latest version number.

If direct/semi-automatic download is not an option for one reason or another, then at least comparing locally stored version number vs. canon (and suggesting users download the more recent version if need be) would be a minimal adjustment.