0025723: Visualization, TKV3d - the center of rotation should be calculated taking...

0025723: Visualization, TKV3d - the center of rotation should be calculated taking into account structure visibility and selection flags

V3d_View::Gravity() - compute scene bounding box only including highlighted objects, if there are any, to provide context-oriented center of gravity.
Skip hidden objects, if there are any visible.
Do not return any value from this method.

Extend command vrotate with new flags -mouseStart and -mouseMove, to emulate rotation by mouse.

Unused argument theDi of VRotate disabled