This will be used by the update check to verify you are running a new
version since last time. In the future, it might even allow to handle
some types of config migrations if ever we update some things in-between
micro releases. Until now, we could only detect minor updates through
the config folder name (and even this was limited as the config folder
can be specified, in which case we would not even know what version the
files were for).

Maybe we could start also warning in cases of downgrading too, which can
break some configuration files (though there is not much we can do about
it other than warn as there is no time machine!).

Last point: if the new config value "last-run-version" doesn't exist, it
simply means the config folder was run for a GIMP before this point in
time/commit. So we just show the welcome dialog.