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).

The new messages:
  Syncing - <X remaining | All synced>
  Pausing - X remaining
  Paused
  Incomplete settings

Create a decorator "ssl_fall_back" which catches a KamakiSSLError,
patches kamaki with the default certificates bundle and reruns the
failed method.

The decorator is used on the "SyncerSettings._get_pithos_client"
method, because this is the first point of SSL failure.

The default certificates bundle is acquired from the package
"certifi", which has been added to dependencies.

Before this patch, settings would be permanently saved only if
the state at the end of the modification was enough for a sync
to start.

After this patch, users will be able to save partial settings,
even if there are essential omissions.

Used to open the parent (containing) folder

In protocol.py, correct the WebSocketProtocol docstring.
In gui/protocol.js create a simple method for debug logs.
In gui/menu.html use the aforementioned method.
In protocol.js make sure the connection is closed at shutdown.
In gui.py, remove the GUI session file even when quitting abnormally
Ignore "exclude" setting for now