Support rich text elements in QSGTextNode

Support rich text elements in QSGTextNode

In order to have the same text rendering mechanism regardless
of the contents of the document, we need to implement rich
text layout elements in the scene graph.

Currently we support the following:
- Text objects, floating and inline (including images)
- Tables
- Lists
- Only solid borders
- Frame background colors
- All the HTML text/text layout/font manipulation

Not supported yet:
- Other border types
- Border radius
- <hr />

Task-number: QTBUG-20917
Change-Id: I4112940e6bc4ad74ae749a727acdd7e6fec3f871
Reviewed-on: http://codereview.qt-project.org/5803
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>