Add support for on-screen keyboard on mobile devices

[mtlynch]

Fixes #46
Re-do of #223, which had to be rolled back due to a regression

[djclueless]

Do you really need all this? Can't you just do sendTextInput(evt.data); ?

  if (
              evt.inputType === "insertText" ||
              evt.inputType === "insertCompositionText"
            ) {
              sendTextInput(evt.data);
            }

[mtlynch]

I'd rather start conservatively and expand if we need to. One in particular that I think will misbehave if we send everything is backspace because it's inputType='deleteContentBackward' and its evt.data is null.

Have you seen this? It's a good tool for exploring input events:

https://w3c.github.io/uievents/tools/key-event-viewer.html

[djclueless]

That is a nice tool, I was just using alert with json.stringify on my phone to check the event.