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.

(cherry picked from commit 99f13f6a)

Conflicts:

The only merge conflict was to port m_reference->permissionsFilename()
back to m_reference->path(), because we didn't cherry-pick refactoring
of FlatpakReference class.