Gizmos can't be used after stop pressed

[lawsie]

Describe the bug
When you press 'stop', all gizmos cease to function

To Reproduce

1. Create a new project
2. Add floor/sky snippet and a mesh
3. Press 'stop'
4. Try to use any gizmo

Expected behaviour
Gizmos still function as normal

[tracygardner]

There is no babylon js scene running when you press stop. So you do need to restart the scene. Definitely a future enhancement to allow the scene to be paused.