[imp] IMP 3.2.3 not compatible with Horde 2.1 (horde 2.1 does not have Lang::setDomain)

[Juha Luoma]

IMP 3.2.3 calls Lang::setDomain() from prefs.php and from lib/IMP.php. 
IMP with Horde 2.1 should call Lang::setTextdomain instead.

  - Juha