Currently the old gweather applet and the new clock applet have nearly identical code to parse Locations.xml. Unfortunately, they both have the same bug, in that they skip over unrecognized start tags, but then they treat the corresponding end tag as if it closed the *parent node*, causing them to end up skipping over huge chunks of the file.

This moves gweather-xml to libgweather, and also fixes that parsing problem.
(I have separate patches to make gweather-applet and clock-applet use the new
API.)