C++: show signal declarations when searching for functions

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>