[TextFlow] Add support for vertical alignment of text and nodes in TextFlow

Currently it is appearently not possible to vertically align text and nodes in TextFlow.

This problem is also outlined here:
http://stackoverflow.com/questions/20759380/how-to-center-an-image-in-javafx


E.g. if you have Texts and Nodes of different heights in the same line, everything is aligned at the bottom/baseline.

It can also be seen here (Sample 4):
https://wikis.oracle.com/display/OpenJDK/Rich+Text+API+Samples

Please add support for it.