Add QQuickTextDocument

Adding QQuickTextDocument as a way to
access the QTextDocument of QQuickTextEdit from C++.

This makes it possible to use a
QSyntaxHighlighter from C++ without exposing more
than one single qml property in the TextEdit.

Change-Id: If1790b591493adcb0b68aef0c8ca706e00450657
Reviewed-by: Alan Alpert <aalpert@rim.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>