Add a popup for JoyCon configuration.

[rowan-m]

trafficstars

• Change the button to load a popup as per https://developer.chrome.com/extensions/getstarted
• Show JoyCon images with currently mapped keys
• Allow clicking on the mapping to select a new key

[rowan-m]

@Rumyra adding a few issues to the repo so I can claim to be organised. I'll pick up #5 so this will have something to connect up to.

[Rumyra]

This is a pretty big issue - shall we keep it as one or break it down - I see tasks like:

[] create interface for pop up [] create interactive elements on pop up [] make pop up pop up [] do upload/download

probs a couple more...

[rowan-m]

For sure, I don't mind a bunch of smaller issues or just PRs referencing this one when they come in.