Add root file support

  - add `root` property support to RamlFile
  - let file browser directive detect root file when raml repository
    gets loaded for first time
  - make sure file browser prefers root file as a selected file over
    first file AND previously selected file as a selected file over root
    and first files
  - let `getIsFileParsable` recognize root file and when such exists,
    make sure only that file is marked as parsable