Redesign the Sign Message / Verify Message interface

[shane-kerr]

We have a "Sign Message" button in the "Receive coins" page, which actually brings up a dialog with separate Sign Message / Verify Message tabs in the GUI.

At the very least we should have separate sign message & verify message buttons which bring up separate dialogs. (It looks like maybe some work was started on this but never finished?)

But possibly more thought into exactly what these do and how to use them is necessary.