Input facilities

We need an interface to promote input events from the application backend into the rendering structures. I see the following components:
- An Input trait, which defines the input handling function
- Abstract input events defined in yage-core/input
- Each backend needs to convert platform specific events they receive into the abstracted ones and pass those on into the canvas (and from there on to the renderer etc.)

Start by reading the abstract input event area in yage-core/input, then trace how platform-specific events enter each backend and how the canvas forwards events toward the renderer. Done means the Input trait, abstract events, backend conversions, and forwarding path are defined across the application backend, canvas, and renderer.

Written by the indexing model from the issue text.