[FEAT] Enable the diagram navigation in the "overlays" demo

[tbouffard]

Is your feature request related to a problem? Please describe.
The BPMN navigation doesn't remove the style applied externally from mxGraph (to hide the overlays) by a call to OverlaysPlugin.setVisible (see #62).

It is currently not activated in the demo because when doing a panning to the bottom of the page, the container height increases.
See the following video 👇🏿.

demo_OverlaysPlugin_support_navigation.mp4

Describe the solution you'd like
Enable the navigation in this demo and update the design to make the panning correctly work.
As for all demo where the BPMN navigation is enabled, also add a "Fit" button to reset the rendering at its initial state.

Describe alternatives you've considered
N/A

Additional context
The panning behavior probably occurs because the height is not detected as a 'fixed' value.
The CSS rules are the same as for the "PathResolver" demo and it recently changed to not used "fixed" height. See #61.
We use flex-grow in the demo of the bpmn-visualization and we don't have such an issue. We should have a look at it.