pointofsale: generated bitcoin url might contain invalid characters

The bitcoin invoice url generated for the QR code might contain illegal
characters because the labels can have any text. In default it already
contains spaces (ie. the naming convention "Invoice - 0001").

The invalid URL did confuse e.g. the Bitcoin Wallet Android app, that
couldn't read bitcoin addresses from the generated QR code.

This is fixed by escaping the label text before displaying it as a
QR code.