build: Install files in the proper systemwide location
Some fixes to the build system that I made in order to make distro packaging easier (which we'll need to ship by default in Ubuntu).
When installing in the system, we need to put the extension in ${prefix}/share/gnome-shell/extensions/. Up until now we were just installing in ${prefix}.
Also, the script to call glib-compile-schemas isn't necessary. meson has this built in. In particular this is not useful when installing systemwide, and meson's support handles this properly. We just need to make the export-zip.sh script do this for the zips we'll upload to extensions.gnome.org.