Add address label to request payment QR Code (QT)

In the Receive 'Tab' of the QT wallet, when 'Show'ing a previously requested payment, add a label underneath the QR Code showing the bitcoin address where the funds will go to.

This way the user can be sure that the QR code scanner app the user using is reading the correct bitcoin address, preventing funds to be stolen.

Includes fix for HiDPI screens by @jonasschnelli.