update QRCodeDialog

• remove unused #include <QDebug> and lblBTC label
• update Bitcoin input field to a BitcoinAmountField to allow Bitcoin unit selection
• use BitcoinUnits::format for the resulting amount parameter in the generated URI (always use BTC as per BIP21)
• move MAX_URI_LENGTH and EXPORT_IMAGE_SIZE to guiconstants.h
• add OptionsModel in AddressBookPage and use it in on_showQRCode_clicked() to pass it to QRCodeDialog
• add OptionsModel in QRCodeDialog to enable display unit updates
• add updateDisplayUnit() slot to be able to imediately update currently set bitcoin unit
• make all labels in the UI-file plain text
• resize dialog to match for an updated layout (fields are now stacked and new field)
• remove unused parameters from private slots
• only enable save button, when QR Code was generated
• show message when entered amound is invalid
• add read-only QPlainTextEdit field to output generated URI