Instead of showing all function declarations when searching for methods,
only show signal declarations. Otherwise, both function declarations and
their definitions would show up, which is not too useful.

Task-number: QTCREATORBUG-9543

Change-Id: Ia81e422e4c53dbc4199b30b1d362f4829b337a7f
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>