CamelIMAPXExtdStore: create CamelIMAPXExtdFolder instances
CamelIMAPXExtdStore: create CamelIMAPXExtdFolder instances

* return CamelIMAPXExtdFolder instances when queried for
  a folder (instead of CamelIMAPXFolder)
* requires some code dupe from CamelIMAPXStore so we
  can allocate our own type
* minor internal cleanup (function param renaming)

GNOME/evolution-kolab,GNOME/evolution-kolab