Bug#790204: gnucash: depends on libwebkitgtk-1.0-0 which is deprecated

[Jeremy Bicha]

gnucash upstream is switching to gtk3 and webkit2gtk in git.

I guess they currently don't expect it to be "stable" until the end of
the year, but a beta version might be good enough for Debian testing
and Ubuntu 17.10?

Thanks,
Jeremy Bicha