data/prefs.ui: Changed the name from prefs.xml, to please intltool and
gettext.
search.ui: Change the name from search.xml, to please gettext and
intltool.
data/Makefile.am: Use the new *.ui files instead of the *.xml ones, so
gettext and intltool are happy.
src/prefs.js: Use the .ui files instead of the .xml ones, and change
the translatable strings from '' to "", to please gettext and intltool.
src/extension.js: Change the translatable strings from '' to "" to
please gettext and intltool.
po/POTFILES.in: Use [type gettext/schema] for the schema file, and
[type: gettext/glade] for the Glade UI files. The colon in the later
is necessary, and doesn't work for the schema (why?). Also, the
translatable strings in the JavaScript files had to be changed from ''
to "", otherwise it doesn't work.

But now we can keep the translations updated by only doing "make
update-po", and we can get rid of the hackish update.sh script.
po/gnome-shell-extension-weather.pot: The file is finally generated by
"make update-po", as it should, but I'm keeping in the repository to
make the life of the translators easier.
po: Update the translations.