qt: add message field to SendCoinsRecipient

Also update URI parsing to fill in this field.
Note that the message is not currently used in any way with the client.
It should be stored with the transaction.
qt: use SendCoinsRecipient for payment request information

This brings some symmetry into the design by using the same object
both for incoming URIs that are parsed as for outgoing URIs that
are formatted.
qt: move bitcoin URI formatting to guiutil

Follow the same pattern as the parseBitcoinURI function.
qt: format bitcoin URI as URI, add copy button