[FEAT] Zoom in and Zoom out with Ctrl+mouse wheel

[NathalieC]

To focus on a diagram part, or move away back to the global picture, a user can press Ctrl+move the mouse middle wheel: the diagram is zoomed in and out, centered on the viewport center.
This should work for any OS.

[tbouffard]

This won't be activated by default, because we are not sure this is needed all the time and this currently don't work well when the enclosing viewport (html div) has not a fixed size. This is the case in our current demo.
A configuration parameter will be available to activate it as for panning.

For floating size viewport, the viewport size is increased on zoom

[tbouffard]

macOS support is deferred to #773