Components to allow to check duplicated metadata titles in the metadata editor and display a message to the user

[josegar74]

A backend service and an AngularJs directive to use in the metadata editor for the metadata title element to verify if the value is used in other metadata record(s) and display a message to the user.

To configure the directive, in the config-editor.xml of the metadata schema, configure it for the title element. For example, for iso19139:

<fields>
  ...
  <for name="gmd:title" use="data-gn-duplicated-metadata-title-checker" />
  ...