allow emit message() in sendcoinsdialog and walletview

- this allows us to use emit message() over MessageBox:: or gui->message()
  calls in sendcoinsdialog and walletview
- move main handlePaymentRequest() functionality back to BitcoinGUI
- move a showNormalIfMinimized() before gotoSendCoinsPage()
qt: ReceiveCoinsDialog and SendCoinsDialog should not take a gui object

They're supposed to be children of the walletview, and are added with
addWidget later on.
Inconsequential change because the object passed was already 0.
Merge pull request #3144 from Diapolo/message_sendcoinsdialog

allow emit message() in sendcoinsdialog and walletview