minor refactoring

darcs-hash:20090123113559-b2873-a330be7bd2bc354ad2adb8e2740b38be1d5f1d6d
sip_im_session: added throttling of sounds and removed distinction between caller and listener mode

darcs-hash:20090123113720-b2873-19b405c5d1870146349e95c65f1f99d7cdf58b4a