FlatpakPermissionModel: Store values of Filesystem entries as AccessMode enum

This completely gets rid of i18n strings for storing values or interfacing with front-end code for model's business logic purposes.

Parsing [Context] filesystems= is now done properly: first reading it as ';'-separated standard XDG List, then parsing with FlatpakFilesystemsEntry and validating the std::optional result. During overrides loading, comparison with existing entries is done using prefix+path part of the filesystem, which is a new method factored out from format() but without access mode prepended or appended.

The addPermission method may be temporarily broken for editing filesystem entries, and default checked state might not always make sense, but we will fix them step by step. Up until now it was unclear what checked state was supposed to represent either.