Zoom with mouse button + motion

[Plonq]

Adds an optional button_zoom field, allowing users to configure a button which, when pressed, will zoom the camera based on mouse movement. E.g. hold right click and move mouse down to zoom out.

Can customise:

• The mouse button used
• The axis of mouse movement used to zoom (X, Y, or both)
• Zoom direction (via existing reversed_zoom field)

closes #128