Remove visibility property from Graphics

Since the concept of 'activity' and 'visibility' is somehow equivalent
on a graphics representation, 'visibility' property is removed from
graphics representations.

When a graphics representation is active, it is visible; when it's
inactive, it's invisible.
Revise comments and update mock objects
Bug fix

When a graphics representation is added to a group, its
activity/visibility should be the combined effects of the group's
activity/visibility and its own activity/visibility.
Merge remote-tracking branch 'origin/master' into feature/graphics-remove-visibility-property
Cpplint and doxygen warning fix
Roll back changes to (Osg)Group
Graphics manager won't check for activeness any more.

Move the responsibility of checking for activeness from
Graphics::Manager::doUpdate() to the derived classes of
Graphics::Representation.
Revise UnitTests
Roll back changes in OsgGroup
Revise documentation
Update serializable stapling demo
Merge branch 'master' into feature/graphics-remove-visibility-property
Rollback changes in (Osg)Group and its UnitTests
Keep ActivenessTest in OsgCameraTests
Remove unnecessary 'setLocalActive()' in Fem3DMeshRenderTest.cpp
Merge branch 'master' into feature/graphics-remove-visibility-property
Merge branch 'feature/graphics-remove-visibility-property' into 'master'

Remove visibility property/concept from Graphics

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/1132553