webadmin: Introduce ListModelSuggestBoxOnlyEditor

Similar classes exist for ListBox, etc. Useful to have a version with
invisible label.

Change-Id: Idc3f9f8c4d3873716f352c3d5065ea78212488b6
Bug-Url: https://bugzilla.redhat.com/1061193
Signed-off-by: Lior Vernia <lvernia@redhat.com>