[Qt] add BIP70/BIP71 constants for all messages and mime types

- also rename current ones to match the new ones
- remove constant from guiconstant.h and add it to paymentserver.cpp

src/qt/guiconstants.h

@@ -38,9 +38,6 @@ static const int TOOLTIP_WRAP_THRESHOLD = 80;
 /* Maximum allowed URI length */
 static const int MAX_URI_LENGTH = 255;
 
-/* Maximum somewhat-sane size of a payment request file */
-static const int MAX_PAYMENT_REQUEST_SIZE = 50000; // bytes
-
 /* QRCodeDialog -- size of exported QR Code image */
 #define EXPORT_IMAGE_SIZE 256