Improve zoom and scroll

[pscheede]

Hi!

When trying out the new experimental "static" export of structurizr/cli, I've stumbled across behavior of the UI that is not ideal, in my opinion. A quick check in structurizr/lite revealed this behavior there as well.

• When dragging a large diagram, the mouse position is not fixed to the diagram. Instead, the mouse moves more than the diagram does.
• Zooming is linear, which means that the first zoom step is perceived to be much bigger than the last steps.

This PR is my attempt at fixing these problems, by:

• removing an (IMO) unnecessary scale factor when dragging the diagram
• changing zoomDelta to zoomFactor and therefore using exponential zooming, which feels much more natural, in my opinion

I also slowed down zooming by mousewheel on the static page, because I felt, that the mousewheel could benefit from double the amount of zoom steps.

Please let me know, what you think, and if there's anything you'd like me to change!