Implement the UI for the new Notifications API.

This currently only shows connection error notifications which have to be
removed manually since an account with a connection error can not currently
reconnect successfully.

Testing Done:
I used a IRC account with an server name that was unresolvable as well as an XMPP account with a wrong password to cause a fatal connection errors.
I also used a IRC account connecting to a local instance of ZNC where I terminated ZNC to cause a server side connection failure.

Bugs closed: PIDGIN-17634

Reviewed at https://reviews.imfreedom.org/r/1512/