[Development] Renaming QQuickCanvas to QQuickWindow

[martin.jones at nokia.com]

The QML team recently reviewed the QQuickCanvas API; a task that was overlooked during QtQuick 2 development.  We came to the conclusion that QQuickCanvas is the wrong name and that QQuickWindow better encapsulates its role and usage.  It also brings the C++ API and the QtQuick.Window API in line.

https://codereview.qt-project.org/#change,30167

There are also a few other small API changes to that API:

rootItem() => contentItem() - like Flickable contentItem and removes confusing overlap with QQuickView::rootObject()
grabFrameBuffer() => grabWindow() - similar to QPixmap::grabWindow()
setClearColor() => setColor() - to match the QML API

QQuickItem::canvas() => QQuickItem::window() - for obvious reasons.

I realize that it is a late change, but it's better to get this new API right for the Qt 5.0 release.  We'll keep the existing API available for a short time to allow time for porting, but it will be removed before the Qt 5.0.0 release.

Br,
Martin.