Converted horde/Data to using Horde_Session

Converted horde/Data to using Horde_Session

No injection or anything - just switching out $_SESSION calls for
Horde_Session calls.

horde/Data in its current incarnation is EXTREMELY tied to Horde
applications.  It either needs to be abstracted more or completely
incorporated within horde or horde/Core.  Additionally, apps should not
be directly accessing the import_data session value.