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.

(cherry picked from commit 6530e94c)