Some projects use separate directories for sources and headers.

An example tree:
*
|-- src
     |-- foo.cpp
|-- include
     |-- foo.h

Allow the user to specify directories for finding out-of-project related
header/source files, in addition to current directory

Task-number: QTCREATORBUG-8883
Change-Id: I57215c8f2feffcc246d0d161798290861bcfcdd4
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>