Introduce new technique to lookup forward headers

We are now able to propose headers in the missing-include assistant,
which were not parsed before (i.e. that are not in the DUChain).

The procedure is simple: Assume we write something like 'QState',
and qstate.h is not yet included nor parsed, in some file foo.cpp.
KDevelop then checks if there is a file called 'QState' in
any of the include paths required to build foo.cpp and then adds it
to the list of possible include directives.

This is possibly useful in other areas, e.g. for the "Create From
Template" feature, as well.

BUG: 287492