Introduce char event, and prevent notebook shortcuts

[almarklein]

This PR:

• Uses a <input type='text'> element to hold the focus (was previously an <a> element).
• This prevents the notebook from capturing key presses and using them for shortcuts, like pressing 'a' to add a cell or 'd' to delete a cell.
• This also allows capturing 'input' events to emit 'char' events, see Figure out how to handle unicode char events pygfx/rendercanvas#28 This is an experimental feature.
• Specify in the spec that key_down and key_up events should not repeat.

cc @kushalkolar

[almarklein]

This prevents the notebook from capturing key presses and using them for shortcuts, like pressing 'a' to add a cell or 'd' to delete a cell.

@kushalkolar have you or your users had trouble with this? I.e. making an interactive thingy with keyboard shortcuts but then not being able to use all keys because they are already captured by the notebook?