Export QTextImageHandler and add accessor for image

Export QTextImageHandler and add accessor for image

To allow optimizations in the scene graph, we export the private
QTextImageHandler class and add a function which will fetch
a given image when required, or return the cached image in the
document resources.

Task-number: QTBUG-20917
Change-Id: If45e51b0d9a267bc198623165e7a2cc1fb2b961f
Reviewed-on: http://codereview.qt-project.org/5227
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>