CamelIMAPXExtdStore: inject our own extended connection manager
CamelIMAPXExtdStore: inject our own extended connection manager

* in the class init() function, delete an existing
  CamelIMAPXConnManager instance and bind a
  CamelIMAPXExtdConnManager instance in its place
* in the class dispose() function, unref our own instance
  after disconnecting and make sure the parent's
  dispose() function does not try the same as well

GNOME/evolution-kolab,GNOME/evolution-kolab