Search for updates

[pmartin]

The application should regularly search for updates (typically, calling a remote URL, and comparing the returned version with the local one), and display an information to the user if an update is available.
=> The user will then have to download the update on a computer, transfer it via USB, ...

As the app is mostly used offline, this should be done at the only time we know we are only: when syncing entries with server. Also : only check if no check has been done recently.

This should be enough, at first.
Later, we might implement some in-place auto-update mechanism; but it might require a bit more work and is probably not quite necessary for the first few releases.