Cleanup of camera code in preperation for shadows

Cleanup of camera code in preperation for shadows
Partition depth in CameraGroup:
Remove the ViewPartionNode scenegraph node. The split rendering of the
scene, done to avoid Z buffer precision problems, is now done by two
slave cameras of the viewer.

Rename FGManipulator to FGEventHandler.

Remove virtual member functions that aren't required for event handlers.

Begin using camera group properties to update cameras at runtime;
Initially only the viewport properties are used.

When no camera group is found in the property tree (the default),
create the properties for one. Expose the default window by name.

Add a test for Boost headers to configure.ac. Boost is now a
dependency.

Remove GLUT and SDL versions of the OSG graphics.