Pause button and menu

[nicklockwood]

This PR adds a pause button to the in-game HUD, and a pause screen menu.

This was not a simple thing to implement due to the separation between rendering and game engine, which makes it hard to map on-screen touches back to the position of controls.

The solution involves a Swift UI-like declarative layout system so that the screen position of views such as buttons can be computed independently of rendering.