Remove superfluous include paths from project files.

A lot of our build system files specify unneeded include
paths. These roughly fall into the following categories:
    a) Paths that are already set in more general files
       such as qtcreator.pri.
    b) Paths that serve no purpose at all, possibly
       left over from earlier versions of the project.
    c) Paths that act as workarounds for wrong include
       statements of the form '#include "xyz.h"', where
       xyz.h is not in the same directory as the including
      file.
This patch removes such path specifications and fixes the offending
include statements from case c).
Tested on Linux, Windows and OSX with qmake and qbs.

Change-Id: I039a8449f8a65df0d616b4c08081145c18ae4b15
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>