Add output highlighting/insert support for Isar and sledgehammer ("Sendback")

[David Aspinall]

To do:

• Add context-sensitive binding for Emacs button in response buffer (use mouse-1, cf GNU Emacs button behaviour and mouse-2 -> mouse-1 rebinding)

Issues:

• The window containing the material goes away as soon as anything else happens. In particular this makes it pointless to have more than one "sendback" link at once. Better would be a permanent pop-up window, if that's possible. This is what is done by the PhoX _pbrpm_ module, we might be able to reuse it here.

• I get problems with the X-symbol markup going crazy. This happens when I try to combine (in out message) a small amount of "sendback" text with other material that contains X-symbols.

[David Aspinall]

Fixed. See also #267.