Selection object, to control different contexts and selections
SelectionCommand and SelectionEvent relies in new selection object
Added selection object to model
Selection test
Updates after API change
Updates tests after API change
SceneView uses selection contexts
pushContext->pushStack, to avoid confusion with the term "context"
Clipboard uses context. Cut test updated
Fix after rebase
Comments addressed.
Merge pull request #668 from anserran/contextmanager

A better managment in selection and context