Simulate user interaction

Provide interfaces simulating user interaction, written from an proper level of abstraction.

- Mouse input (clicking)
- Keyboard input (clicking)

Out of scope for now,

- Mouse input (drag-n-drop). Simulating real drag-n-drop effectively would _probably_ require a coordinate system, which again requires _some_ kind if visual rendering.

The issue names no files, tests, or entry points. Start by locating the existing Go browser interfaces and their test entry points; done means mouse clicking and keyboard input can be simulated, while drag-and-drop remains out of scope.

Written by the indexing model from the issue text.