Install included RELAX NG schema and make it automatically loadable by Emacs.
Update spec file to install RELAX NG schema.
Add appdata-rng-init.el to .gitignore.
Multiple name and summary elements should be allowed, since they may have
different languages.
Generate appdata.rng from appdata.rnc at build time.
Rename schemas.xml to schema-locating-rules.xml to make its purpose more clear.
Use embedded Schematron in RELAX NG schema to check for duplicate name and summary elements.