Is it possible to add mobile friendly pinch to zoom?

[fkloepper]

Currently zooming on mobile is only possible through the zoom-button on the bottom right.
It would be nice to be able to add a pinch-to-zoom functionality

Edit:
Being able to zoom with the scroll wheel on desktop would also be a good addition

[MaxwellEdisons]

There is also this need. In addition, it would be better if it can be automatically reduced to the size of the current window during initialization.

[rom1dep]

Not sure how "mobile friendly" it is, but this PR makes touchpad pinch to zoom "do something" at least: #277