Add Keyboard Support to Canvas

### What is the problem or limitation you are having?

The Canvas widget is has no keyboard handling. It would be useful to be able to do things like scroll a plot using arrow keys or capture simple keyboard input for games.

### Describe the solution you'd like

Add a callback for keyboard events on the Canvas widget. The callback should be given the key pressed as a Toga Key expression, along with any modifiers, and ideally the text that results from that key combination with the current keyboard layout, if any.

### Describe alternatives you've considered

Leaving it as is, and implementing a more robust input handling system at a higher level that is independent of the Canvas widget. For example the proposal doesn't handle composed keys or entering text in Chinese/Japanese/Korean. These require additional special handling which should be its own subsystem (see eg. hoe Gtk handles composing characters).

### Additional context

This is related to #3994 in so far as the Canvas element can have `keypress` handlers. Looking at how HTML/Javascript handles key presses may give a guide to the information that is passed to the callback.

Start by reviewing the Canvas widget and the related #3994 discussion, then compare the requested callback data with HTML/JavaScript keypress behavior and Toga Key expressions. Clarify how modifiers, resulting text, composed keys, and CJK input are scoped; done means the supported keyboard behavior and its limitations are implemented and documented.

Written by the indexing model from the issue text.