UI improvement: use a button instead of double-tap

[afiskon]

First of all, thank you a lot for a great application!

I noticed a little issue with the UI. The problem is that double tap doesn't always works reliably on Android. Thus sometimes you do a double tap, wait assuming that the application is pairing with the device, realize that double tap didn't work, repeat a few more times until it finally works.

I suggest to use a button instead of double tap and make it inactive during the pairing. Thereby the UI will work more reliably and user will see if the scan was started or not.