Issue #3294269: Fixes of Drupal 8 compatibility issues after adding...

Issue #3294269: Fixes of Drupal 8 compatibility issues after adding non-existent service "file_url_generator",
simply moving the needed @file_url_generator service methods in the inner @geofield_map.marker_icon service,
with @todo notes to refactor all this in favour of only Drupal core 9 & 10.