When checking for updates, write the new timestamp and version info
to Gimp::edit_config, instead of Gimp::config, and rely on its
auto-save mechanism to save the changes, in order to avoid having
to save Gimp::config on exit, which overrides modified settings
that require a restart, such as the UI language, as they are only
reflected in Gimp::edit_config.

This fixes the bug, but can cause the new update info to be
discarded if the Preferences dialog is open while checking for
updates, and is subsequently canceled.  Ideally, the update info
should live in a separate file, rather than in gimprc.