Switch to libxml2 for output of glifs and plists in the U.F.O. driver.

This addresses issue #1398. At the moment, the only functional improvement is in indentation, but this is an important first step towards correct handling of foreign data.

It does not seem to be the case. Looking at [the spec](http://unifiedfontobject.org/versions/ufo3/glif.html), GLIF files don't have the plist extension and the example they provide at the bottom does not have DTD either.