Add Include for Undefined Identifier: Check also the include paths

1. In a clean Qt Creator session (no other projects loaded), create a GUI Hello World application without a ui form.
2. Insert some "QPushButton b;" in a method
3. Trigger Quick fixes - there is no offer for including <QPushButton>

Currently the fallback is to look in the Locator if we can find a class with that name. If other projects are open that pull in such a class, the quick fix will be offered.

Enhance this to look into the include paths!