Create rectangle nodes in the text editing through the context

Unfortunately we can't re-use the QSGSimpleRectNode, as it doesn't provide us
with virtual methods to move it's creation into the context. But's since it's
only 20 lines of code anyway, this is still a nice cleanup. And it also allows
the re-use of any optimizations in the renderer for QSGRectangleNode.

Change-Id: I957777fbbeb0a994a9c257baf3bfe87fce8cc9e8
Reviewed-by: Lars Knoll <lars.knoll@digia.com>