Dear Maintainer,

Steps to reproduce:
* set delete messages to mark as deleted only
* add a delete button to the top toolbar
* select the last message in a folder, so that automatic advance
  to the next message isn't possible
* click the top delete button

Notice that the label stays as 'Delete', though the message is now
marked as deleted. Further clicks will toggle the button label and the
state of the message, so the label continues to indicate the opposite
of the action it will cause.

Richard