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.