generate-plugin-docs.pl was assuming that it was being run out of
srcdir, and so failed in srcdir!=builddir builds (such as "make
distcheck"). Fix that.
Also, update .gitignore
The scripts extracts plugin description from document comments for particular
properties and builds a XML file out of the data. The XML file can be used
later for generating manual pages or other documentation.
Unfortunately, gtk-doc won't allow descriptions that would be separated from
the main gtk-doc stuff. But it is still useful to have plugin description bits
co-located with property definitions. We use our home-grown comments and parse
them ourself. Afterall it's not that bad, and in addition it brings us a
freedom in shaping the comments to our needs.
