Add gestures to shortcut overlay

## Problem

Hard to know what gestures do what. Might be obvious to the general user but this is hard for users who are new to using laptops/newcomers in general.

## Proposal

Add gestures to the shortcut overlay. Suggested list of gestures to be added:
- Multitasking view
- Switch workspaces
- Scroll
- Delete current workspace (I'm not sure if there's such a gesture)

## Possible issues

- Unlike shortcuts, it's hard to visualise and show a nice label for gestures. It probably has to be described i.e. "Swipe up with four fingers", etc.
- Looks out of place. Everything on the shortcut overlay are keyboard shortcuts. Having a trackpad gesture seems really out of place here.

## Prior Art

None