Isabelle find-theorems form

[David Aspinall]

Contributio from Tjark Weber:

We have a tested and (hopefully) stable version of the search form now, that I think is ready to be integrated with the CVS version of ProofGeneral.

I don't have write access to the CVS repository, so I'm sending the files to you instead. The attached archive contains two files: ProofGeneral/isar/isar.el and ProofGeneral/isar/find-theorems.el. The latter is new, while only a few lines of the former have been modified to integrate the search form.

Please let me know what you think. (Feel free to take a look at the code and improve it if you like, of course.)