This patches add handling for unseen messages counter, and last message received. The
counters are retrieved from DB on startup, they updated in-memory. Frontend are expected
to have their own in-memory counter too, the backend one is used for re-sync (e.g.; on page
reload).
Bridge methods are added to retrieve the state, and indicate which message has been seen
last by user. A signal is sent on counter reset to all frontends can synchronise.
This is currently local only, but it should be compatible with a future XEP-0490 (Message
Displayed Synchronization) implementation.
A `profile_ready` signal is now sent, so all frontends can know when a profile is
connected, and we don't have to wait for `connect` bridge method and risk a timeout.
If the backend was not closed properly, the former socket may still be present. This was
preventing the backend to start again, while now the old socket is removed in this case.
This is a huge configuration refactoring.
The configuration system now uses Pydantic settings, with better validation, better
support for environment variables and other goodies.
The main config files now use TOML instead of INI, this allow for better handling of
complex types, notably dict.
Plugins, components and frontends can declare their config settings in a class which will
be imported dynamically.
