Refactor Collection from melib to hold what folders have what envelopes.
Frontend accounts will now have a FolderEntry for each logical folder
and will unify many Account fields into one and eliminate a lot of
duplicate/dead code.
Use FolderHash directly as a cursor type for folders within an account
isntead of having a usize (being the order of the folder within the
account) and figuring out the folder_hash everytime it's needed.
Add OfflineListing for offline accounts and AccountStatusChange event.
Subscription status was checked/modified in various places, whereas now
the universal truth is the `BackendFolder::is_subscribed()` method set
by the backend when a folder is created. The `Account` struct passes a
closure to the backend constructor that determines whether the folder is subscribed or not according to the user configuration.
- If subscribed_folders field is empty, then all folders are subscribed.
- OR check explicit folder configuration
- OR check if folder path matches to a glob in subscribed_folders.
Merge ui crate with root crate.
In preparation for uploading `meli` as a separate crate on crates.io.
Workspace crates will need to be published as well and having a separate
`ui` crate and binary perhaps doesn't make sense anymore.
Manos Pitsidianakis