Scroll can pan or zoom : useful for touchpad

[Dvad]

First of all, thank you for writing this wonderful library!

I would like to propose this changes about the behavior of scrolling on plot.

Before, scrolling was exclusively for zooming.
But on modern touchpad (e.g. macbook) mouse wheel event is sometime a 2 dimensional vector (vertical and horinzonal) and hence can be use for more fine grained control.

This implements optional panning and zooming with the mouse wheel in a satisfying way for such touchpad. This adds a few options controlling:

• if the mouse wheel panning or zooming by default
• the modifier key toggling between the two behavior (none for behavior non togglable)
• the rate of panning and zooming
• if both axis should be controlled together with the vertical axis or separated with the horizontal and vertical component.

In particular, the default behavior does not change.

Would such a change makes sense in your opinion?