Use Backend signals instead of leap.events for notify the GUI.

Right now, we use leap.events for signaling some components' events like soledad, keymanager, imap, smtp. Use backend signals to notify the UI.

➜ git grep 'leap.common.events'
app.py:from leap.common.events import server as event_server
crypto/srpauth.py:from leap.common.events import signal as events_signal
crypto/srpauth.py:from leap.common.events import events_pb2 as proto
gui/mail_status.py:from leap.common.events import register
gui/mail_status.py:from leap.common.events import events_pb2 as proto
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mail_status.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mainwindow.py:from leap.common.events import register
gui/mainwindow.py:from leap.common.events import events_pb2 as proto
gui/mainwindow.py:        :type req: leap.common.events.events_pb2.SignalRequest
gui/mainwindow.py:        :type req: leap.common.events.events_pb2.SignalRequest
platform_init/locks.py:from leap.common.events import signal as signal_event
platform_init/locks.py:from leap.common.events import events_pb2 as proto
services/mail/conductor.py:from leap.common.events import events_pb2 as leap_events
services/mail/conductor.py:from leap.common.events import register as leap_register
services/mail/conductor.py:        :type req: leap.common.events.events_pb2.SignalRequest
services/mail/conductor.py:        :type req: leap.common.events.events_pb2.SignalRequest
util/log_silencer.py:        'leap.common.events',

➜ git grep 'from leap.common.events import register'
gui/mail_status.py:from leap.common.events import register
gui/mainwindow.py:from leap.common.events import register
services/mail/conductor.py:from leap.common.events import register as leap_register

(from redmine: created on 2014-05-28, relates #6359 (closed), relates #6360 (closed), relates #6099 (closed))