Auto-hide balance while entering request amount

Bitcoin Jason reports merchant users getting confused while entering a request amount between the request amount displayed below the QR code and their balance displayed above it.

They may not know of the option to hide their balance, or think to disable it while requesting payments.

We could add an "auto balance hide" when requesting an amount, which can also be optionally set to "off" for power users but defaults to "on".

Edit: After discussion: instead maybe better to pop up a full screen request modal using the "Request amount" button. Can also be triggered by long pressing QR code. Follow up: Make a "Selene Point of Sales" mode which switches on that screen as the default, and has some nice payment accepted animations etc.