QQuickCanvas renames

QQuickCanvas is now called QQuickWindow
QQuickCanvas::rootItem is now QQuickWindow::contentItem
QQuickItem::canvas is now QQuickItem::window
QQuickItem::ItemChangeData::canvas is also renamed window
QQuickCanvas::grabFrameBuffer is now QQuickWindow::grabWindow
The functions related to the color property have dropped the clear from
their names.

The first three changes have interim compatibility measures in place to
ease the transition.

Change-Id: Id34e29546a22a74a7ae2ad90ee3a8def6fc541d2
Reviewed-by: Martin Jones <martin.jones@nokia.com>