[Development] Header file cleanups

[lars.knoll at nokia.com]

Hi,

I've seen a few examples where people are cleaning up includes in header
(public or private but exported to other modules) files, as e.g.
http://codereview.qt-project.org/#change,18095.

While they are correct in theory, they cause problems in practice and
provide close to 0 (in fact negative) value to Qt. The only advantage is
some purity of our code and maybe a .1% speedup of compilation. This does
not outweigh the negative side effects:

* They break SC needlessly for people porting from Qt 4.x
* They break downstream projects and some people (including myself) are
wasting quite a bit of time fixing up these breakages instead of fixing
actual bugs.

So please *don't* do these changes. They are simply not worth it.

Feel free to do these cleanups in .cpp files or private headers that
aren't exported/used by other modules.

Cheers,
Lars