Add BOM and split master pom to '-metadata' and  '-with-dependencies' poms

 * similar approach as used by KIE to avoid the chicken-egg issue with bom
 * tested as 'mvn clean install' using empty Maven repo
 * child modules use relativePath to specify parent pom

The POM (directory) structure looks like this:
uberfire-parent-metadata (extends jboss-parent)
  |-- uberfire-bom (extends uberfire-parent-metadata)
  |-- uberfire-parent-with-dependencies (extends uberfire-parent-metadata)
  | --...other uberfire modules extends uberfire-parent-with-dependencies (uberfire-api, uberfire-backend, etc)