At the backend level, some syncing operations may fail and restart
again. Failed operations will produce a SyncErrorMessage, which is
consumed by the UI, increasing the corresponding "failed" counter.

This allows the GUI to correctly calculate the number of synced
and unsynced files.

All status codes are defined in an external JSON file named
"ui_common.json". This file is loaded by the UI components if they
must exchange statuses.

Also, the protocol for propagating the status of the server has
been modified. Instead of using separate flags to denote if the
server is in some status or not, we now use a status code which
must be defined in the common JSON file.

In the present commit, the WebSocket server is managing status
codes in that fashion, and the GUI is able to understand the
responses and adjust its behavior accordingly.

Refers to the "heart" relation of the "session.db" database, used
by UI to orchestrate multiple clients.

All menu icons are now 20X20 pixels and they are instructed to not
behave as template icons, allowing them to be rendered with colors

Instead of updating the "heart" database from the context of each
connection, update it from the context of the helper session.
The connections will still poll the database to decide whether
they are going to stay open and still have the authority to empty
the database when there is a shutdown request.

Removed: launch_daemon, stop_daemon
New: start, shutdown, pause

start: Start syncing. If the daemon is not running, launch it
shutdown: Cleanly shutdown everything in Agkyra (backend, GUI,
    daemons)
pause: Set the running Agkyra session to "paused" mode

To shutdown all UIs, one has to "stop" the heart of the UI session.
The heart of a UI session is a database relation in the
"session.db" database. It is emptied when a shutdown message is
received by a running WebSocketProtocol instance.

All other connections periodically check the heart and update its
timestamp. When the tupple containing the session id is empty, all
connections of the same sesion die.

The WSGI server, which is running as a member of a SessionHelper
instance, is shut down by a simpling threaded method launched by
the SessionHelper before running the server. This method is called
_shutdown_daemon and is polling the database every 4 seconds. When
the database is empty, it sends a shutdown signal to the WSGI
server.

These changes affect the way an Agkyra daemon is launched, as well
as the "launch_daemon" and "stop_daemon" CLI commands.

These commands can be used to launch and stop the agkyra daemon

Operate internally only with unicode. Talk to the filesystem with utf8
encoded strings and then decode the retrieved paths to unicode. Refuse to
operate if the assumed filesystem encoding is not utf8.

To achieve this, a new method "wait_session_to_load" is
implemented in the SessioHelper class, which blocks the execution
until a server session (any server session) is loaded.

The SessionHelper does not create a new session on instantiation.
Now the caller has to explicitly ask the helper to load a session
or create a new one.

Change GUI message to "Connecting..." on start up.
Debug-log the database clean up when closing helper connection.

The helper will catch all remote client errors and inform the UI
instead of terminating.

At the GUI side, on 401 UNAUTHORIZED errors, the settings window
will open, prompting users to check their settings. This behavior
will be repeated whenever there is an authentication error (e.g.,
when the token expires).