webadmin: TypeAheadListBox to display default text

Adjusted the widget to display default grayed-out text when it has no
content.

Change-Id: I18d43efadde607867be440af73a9d67a14c293e0
Bug-Url: https://bugzilla.redhat.com/1086259
Signed-off-by: Lior Vernia <lvernia@redhat.com>