Implement a dialogue window which is pop up when the directory or
container have been removed. The proposed solution is to create
the missing piece and reset the syncing process. If the user
accepts, the proposed solution is enforced. Otherwise the system
stays in a non working mode, where the user can fix the problem by
handling the settings.

GUI parts adjusted: menu, settings.
Greek is not used yet.

Notification messages should not be stated inline, but loaded from
external data files. For that purpose, a new directory "ui_data"
is created, to store data files for the UI.

Currently, there is just one data file "common.json", containing
as dictionary of protocol statuses and corresponding notifications.

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.