app: serialize and deserialize extensionrc from GimpExtensionManager.
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.