[zoom plugin] no mouse follow

[vdegenne]

I leave that here in case you want to help me collaborate.

It's just a plain implementation of no mouse follow for the native zoom plugin.

The main issue left is mouse events are not properly scaling with the zoom factor.

should implement #3027

note: Also no follow should be a toggle flag, it's on the todo-list.

[soreau]

This code looks a lot more complicated than I'd expect for this feature. The way I see it, you should be able to install an activator binding which toggles a boolean and records the cursor position. Then, if the boolean is set, use the saved cursor position instead of the actual cursor position. Bonus points to add a transformer that smooth-moves from saved cursor position to actual cursor position when the keybinding is toggled 'off'.