darcs support

[Guillaume Hoffmann <guillaumh gmail com>]

Dear Meld hackers,

I see that since version 3.15, the Darcs version control system is no
longer supported, due to a change in the API of Meld, and the cost of
maintaining Darcs compatibility.

I'd like to propose rewriting Darcs support for Meld. Since the 2.10.3
release of Darcs, there exists a command `darcs whatsnew
--machine-readable`which is quite easy to parse.

One thing I would like to get right are file moves, which are natively
supported in Darcs. I saw that Meld supports file moves in Bazaar. Is
it a good idea to copy the Bazaar module and adapt it to Darcs?

Thanks!

Guillaume Hoffmann