Remove the PostScript plugin, clean up export filters and WMF exporter
Thanks for reviewing/merging my earlier MRs! I'll probably have a few more ready soon, but I'm not sure when.
This removes the PostScript plugin, since Cairo can do everything instead. An EPS filter based on Cairo replaces the only thing the plugin provided that wasn't already available.
This removes a bunch of cruft from the WMF plugin, such as pre-NT Windows support. It also removes the dummy GDI plugin, since it didn't really work anyways.
It also drops non-GTK printing support, since GTK is pretty good at that now, even though it's only been sixteen years. With the plugins removed, no-one provides the callbacks anyways.
Lastly, and more subjectively, some export filters have been reorganized, with the Cairo SVG and CairoScript filters removed and some others renamed. See the commit itself for more information.