[RFC] add color to PlaceNode and allow to blend texture with material color

Hi, 
In order to be able to assign a color to a group of PlaceNode, I've experimented some code changes.

We should be able to choose a material color for PlaceNode now.

The annotation sample have been updated in order to demonstrate the new feature.

A sample picture of the result is available here http://paul.chavent.free.fr/osg/osgearth_annotation.png

Could you tell me if it is a good solution please ?
Should i definitely use custom shaders like described here (http://osgearth.readthedocs.org/en/latest/developer/shader_composition.html) ?

Thank for your advices.

Regards.

Paul