Add .on_keyup method, similar to .on_click

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I want to do a search-as-you-type interaction. Currently I'm sending a Command with the query using a Button, so I'd like to change that to sending a Command every time a key is pressed. I also want to intercept keyboard "enter" events and use that to send a different command.

Would it be appropriate to implement this in a similar way as the `.on_click` method? I imagine there's also a way to do this with lenses but I'll admit I haven't fully explored that yet. That wouldn't help with intercepting enter, however.

Start by examining the existing `.on_click` method and the keyboard event handling paths. The work is done when keyup events can dispatch a command for each key press and Enter can be intercepted to dispatch a different command.

Written by the indexing model from the issue text.