qt: make docs, demos and examples optionnaly available

As the only option before was to don't include them at all.
qtcreator: Update to 2.7.1 and improvements (merge #579)

- Qt is now directly overriden in QtCreator's definition.
- qt48Full can be used in a development environment to enable access to
docs demos and examples.

NOTE: It may be required to remove older configuration files located in
~/.config/QtProject and at ~/.config/Trolltech.conf to reconfigure
qtcreator with the correct paths. I have found /nix/store/... paths in the
following files:
$HOME/.config/QtProject/qtcreator/toolchains.xml
$HOME/.config/QtProject/qtcreator/qtversion.xml
$HOME/.config/QtProject/qtcreator/helpcollection.qhc

At the first launch, be sure to check your qtcreator's configuration by
selecting Tools->Options. The most important parts are the Kits and
QtVersion tabs located in the Build & Run section.