DevTools: better visual feedback for hovered objects is Layers3DView

- rename WebInspector.Layers3DView.ActiveObject to ...Selection;
- introduce proper constructors and isEqual methods for Selection;
- generalize hover/selection handling logic for different objects;
- make hovered (but not selected) object bluish

Review URL: https://codereview.chromium.org/711423002

git-svn-id: svn://svn.chromium.org/blink/trunk@185203 bbb929c8-8fbe-4397-9dbb-9b2b20218538