Make QSGTextNode back-end for QML's TextInput and TextEdit

Use the general QSGTextNode class as back-end for all text
elements in QML to make all text elements look the same and
use the same text rasterization back-end. This requires a
few rewrites in the text node to support e.g. selections.

Crashes seen with threaded renderer in TextEdit and TextInput on
Mac are also fixed by this.

Reviewed-by: Jiang Jiang
Task-number: QTBUG-18019, QTBUG-20017
Change-Id: I4207faf180c83422e5f8b726741321af395bd724
Reviewed-on: http://codereview.qt.nokia.com/2865


Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>