Implement touchpad gestures for zooming in/out the canvas

Merged Povilas Kanapickas requested to merge p12tic/gimp:touchpad-gestures-canvas-zoom into master

This MR implements support for touchpad pinch gesture to control zoom within the canvas.

A separate zoom type has been added for such pinch gestures since smooth scrolling uses a delta multiplier to reduce zoom speed. We need a proper mapping between the distance change between fingers and the zoom of the image, so no multipliers are needed in this case.

Related to issue #717.

Edited by Povilas Kanapickas

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