Improve usability and accessibility of the notepad operations
* Add a separate Import button instead of re-using the Sign/Encrypt button. For one, this allows the user to encrypt a public key block. Moreover, buttons that magically change their meaning are bad for accessibility. * Update the three crypto operation buttons in one place. * Disable the Verify/Decrypt button if the notepad is empty.
- GnuPG-bug-id: T6188