FlatpakPermissionModel: Rewrite override data management, drop manual string puzzle

Nuke m_overridesData string and all manual string operations associated with it, in favor of using proper data structures like KConfig and FlatpakPermission. Unparsable entries are preserved in a new special map now, and get written back to support forward compatibility with possible future extensions to flatpak-metadata(5) format.

This commit enabled "OFF" (a.k.a. "deny", !) access mode for filesystems, since now backend code finally able to correctly handle 'enabled' state of filesystems permissions as well. Default mode for disabled-by-default standard filesystems is read-only now.

For the purposes of comparison and some predicates like isDefaults, disabled non-'Simple' entries now ignore their values, as for them the disabled state means that they are actually marked for removal -- a concept that we currently don't represent in the UI in a very obvious way though.