We only save the active state of extensions so that we can reload all
extensions same as they were at previous exit. All other data are saved
as per-extension metadata and should not be saved in the rc file.

If an extension is not listed in extensionrc, we run it by default if
this is a system extension (so that new core extensions by the GIMP team
are run when installed after an updated), but not when they are
user-installed extensions.