Cannot pinch to zoom on touch screens in graph view in GUI

[jfrost-mo]

Description

A common interaction pattern in touch interfaces is pinching to zoom. This does not work in the graph view of the cylc GUI.

Reproducible Example

1. Open a workflow in cylc GUI on a touch screen device, like a touchscreen laptop or a smartphone.
2. Open graph view.
3. Try to zoom via touch.

Expected Behaviour

Can zoom with touch controls, in a similar way to most other apps. Google maps is a good example of how this scrolling should work.