GIMP will now process the remote gimp_versions json file to look if one
is using the last version of GIMP. This initial code doesn't act up yet
on this information. This will come in further commits.

Here are the characteristics:
- Since this requires internet access, a new checkbox is available in
  the Preferences dialog, allowing to disable version checks. Note that
  it is enabled by default as it is an important security feature, but
  it has to be deactivatable.
- The remote access is done as an async operation because we don't want
  it to block the startup in any way (for whatever reason). Also it
  doesn't output errors if it fails to not be a bother (you don't
  technically need internet access for an image program).
- We don't check at every startup. At each successful check, we save a
  timestamp to prevent too frequent useless checks (I set it the timer
  to a week or more for now).