Cache is not useful in the context of the present application.

If some settings are added though the GUI, the client will
save them on exit.

The mechanism is based on syncing message consumption, which works
only while the client is syncing (therefore, not in paused mode).

When GUI consumes a "SyncMessage", an "unsynced" counter is
increased, while a "AckSyncMessage" causes a "synced" counter to
increase. The two counters are pushed to GUI regularly enough.

This patch refers to the settings update on closing the settings
window.

If some settings are missing (e.g., not found in the configuration
file), agkyra GUI will run and let the user complete them through
the "setting" menu option. If the default_sync, sync or cloud
options are missing, they are created automatically.

Settings are saved only if the user opens the "settings" window.

When a user clicks on "Get credentials", a new window appears
where users can authenticate themselves. It is a remote window
implementing whatever mechanism the target cloud implements.

When the authentication window opens, the "Get credentials"
button disappears.

After the authentication is complete or canceled, the following
operations take place synchronously and the order they appear
bellow:
- if the authentication window is open, close it
- remove all cookies related to the account and pithos UI URLs
- Reactivate the "Get credentials" button.

This patch is related to the endpoint URLs needed to make various
UI operations i.e., login and vising the pithos page on a browser.

The helper used to provide the GUI with all URLs necessary, while
now they are retrieved by the GUI itself, with the tokenless
request "POST <auth_url endpoint>/tokens".

Settings are loaded and updated in protocol.py
If the client is not ready yet, some GUI options are not gray
and unclickable until the client is ready.