BMC#15342: Disable Add button until user selects valid IM info
BMC#15342: Disable Add button until user selects valid IM info

This should be done using the active property of the Button instead of a
check on onClicked.  This fixes the previous commit.

Signed-off-by: Kaitlin Rupert <52c295f769a5ee1acd0461c5e936aac51b08fa06@intel.com>

meego-tablet-ux/meego-app-contacts,nemomobile/qmlcontacts,nemomobile/qmlcontacts,meego-tablet-ux/meego-app-contacts