[Qt] Fix build against Qt5 after refactor of widgets out of QtGUi

[Qt] Fix build against Qt5 after refactor of widgets out of QtGUi

QWidget and friends now live in the QtWidgets library. We update
includes in implementation files and private headers to us the
non-module-prefixed path, and leave the lookup for the include
path. For public headers we have to ifdef the includes as the
user might now have the modules we need in his QT config.

Finally, QSGCanvas is no longer a QWidget but a QWindow, so we
have to update our code and use windowHandle() for setting the
parent relationships.

https://bugs.webkit.org/show_bug.cgi?id=68687

Reviewed by Andreas Kling.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96108 268f45cc-cd09-0410-ab3c-d52691b4dbfc