Convert osg::Geode to osg::Group in dae-plugin

[unelsson]

This is more of a suggestion/idea than direct pull request, and may require further work. I hope for feedback, and suggest reviewing the code before merging, and considering whether this kind of change is helpful.

What it does: Converts osg::Geode, used by the Collada plugin, to osg::Group. I've understood that Geode is deprecated, so this should be a step forward. It does fix a few Collada-related problems with project OpenMW, particulary the ability to pick objects with mouse click (collision related?), and I haven't at least yet encountered any regressions.

However, I have limited knowledge of how the scenegraph should actually be handled, and if this is a decent way to do it.

[psi29a]

I should also mention, that some of these collada issues came up when doing imports/exports with Godot's better collada blender plugin. Seeing that the dae osgPlugin stumble made us look at the plugin more carefully.

This should probably go into master to if applied. :)

[psi29a]

@robertosfield how do you feel about this?

[psi29a]

@openscenegraph We've have had this being used widespread and the patch is already being used in OpenMW's PPA, Ubuntu and Debian latest releases on top the official 3.6.5 release; can this be included officially upstream?