add text to verify message window

[pppest]

I tested sign and verify message between v1.2.1.0-5e14268 and v1.2.1.0-23982400b (current develop afaik)
works as expected, but it would be nice if the verify pane/window had a "signature" title like the sign message pane/window to avoid confusion. and if possible add text to show that the big text area is for the message. just to make it clear.