Change doc build on windows

Do not load qt_docs.prf in the qt module.
Rename qtenginiocpp to qtenginio because the module forces that on us.
Instead of qtenginio for the overview docs we now have qtenginiooverview.
None of this is user visible, so it will be fine.

Change-Id: I77bdcb7f4784323e52b79cbe543e16ad4593e533
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>