Metadata editor - support tooltips for thesaurus selector

[josegar74]

Support tooltips for the keyword selector widget. After the change:

The tooltips should be configured in the schema labels.xml with the name attribute as the the thesaurus key.

  <element name="external.theme.httpinspireeceuropaeumetadatacodelistSpatialDataServiceCategory-SpatialDataServiceCategory">
    <label>INSPIRE Service Classification</label>
    <description>The keywords are based on the geographic services taxonomy of EN ISO 19119. This taxonomy is organised in categories, the subcategories defining the value domain of the classification of spatial data services.</description>
  </element>

This is not optimal, as thesaurus can be used in different schemas, but the current code to get the tooltips in the editor is linked to the schema of the metadata being edited.