cmake: Generalise xsd-fu invocation

- Add xsd_fu_single function to introspect xsd-fu for generated files
  and dependencies for a given command, and generate a cmake custom
  command with OUTPUT and DEPENDS set using this information
- Add xsd_fu_header and xsd_fu_source functions to generate either
  headers or sources using xsd_fu_single
- Add xsd_fu to generate both headers and sources using xsd_fu_header
  and xsd_fu_source
- Convert the ome-xml and bioformats components to use the above
  functions to create rules for header and source generation
cmake: Document xsd-fu cmake variables