Screen sharing feature UI adjustment and code refactor

[ttycelery]

In this PR, I tried to tidy things up especially the UI. As a follow-up of #9, in sum, this PR have done the following things:

• UI adjustment: added a select box for selecting a QR source and moved the auto submit checkbox to the bottom of the page
• Auto submit checkbox state is now persistent by utilizing localStorage
• Refactored praesentia.js
• I also re-organized the static files for nimiq/qr-scanner as it looks quite messy (this should be in another PR, but I guess that's okay)

[ttycelery]

What do you think? @Noth3r

Feel free to test and/or give suggestions to it.

[Noth3r]

Nice PR. It works perfectly. But I have a little suggestion to stick with the createQrEngine because in my pc if I create the engine manually, my chrome has lower CPU usage. How about yours?

In nimiq/qr-scanner docs

As an optional third parameter a manually created QR scanner engine instance to be reused can be specified. This improves performance if you're scanning a lot of images. An engine can be manually created via QrScanner.createQrEngine(QrScanner.WORKER_PATH) (async). By default, no engine is reused for single image scanning.

[ttycelery]

@Noth3r Nice catch, I didn't even know that. I can't test its performance, but we are definitely scanning a lot of images (when screen sharing) and are having performance issues. I also noticed that by not using a reusable worker, the browser requests the worker file every time it scans a frame or an image (I think this is what causes the higher CPU usage).

Let me wire up the changes.

[ttycelery]

Does it look good to you? @Noth3r

[Noth3r]

Yep works perfectly. Nice one @p4kl0nc4t 🎉

[ttycelery]

Merged, thanks!