Context menus opened on window titlebars or the desktop should use icons

## Problem

Having to read the text of the items action is slower than a symbolic icon by orders of magnitude. Just having a flat box with the same corner radius of a window makes it look like a window to new users as mentioned in [this issue](https://github.com/elementary/stylesheet/issues/918).

## Proposal
Decide whether to use symbolic or fullcolour icons and implement them in Gala and everywhere else.

## Prior Art