Potential optimization document listeners

[g-rodigy]

Describe the bug
Not a bug.
Scene use Input class (src/viewer/scene/input/Input.js) to add listeners like click, mousedown, etc. Plugins such NavCubePlugin, SectionPlanePlugin and other creates own Scenes with added listeners again that never use them.
As a result, several identical listeners are created that are never used.
For example NavCubePlugin has only one place where used scene.input

xeokit-sdk/src/plugins/NavCubePlugin/NavCubePlugin.js

Line 123 in d1fbab8

this._navCubeScene.input.keyboardEnabled = false; // Don't want keyboard input in the NavCube

When need global scene events can use something like viewer.scene.input

To Reproduce
Create Viewer with plugins NavCubePlugin or other plugins that use their own scene.