Port to GSettings

Taken from https://bugzilla.gnome.org/show_bug.cgi?id=650193

I cleaned up the original patch by Pablo a bit, fixed the build and intltool extraction, and added gettext domain to the schemas. I also made the schemas more idiomatic by not using the deprecated /apps/ namespace and changed type of zoom method to enum. Additionally, I removed the GConf migration since gconf keys cannot be easily converted to enums; it should not matter, though, as the settings are only three tiny presentational preferences.