Bug fixes

Support for dynamic Icon Size with Replacement Patterns/Tokens
Definition and application of LeafletService public function setFeatureIconUrlSizeIfEmpty, to properly set IconUrl and Shadows Urls Images sizes.
Support for className for Icons
Support for weight/zIndex-ing ordering of Features/Markers
Addiction of a new (better named) event 'leafletMapInit' for every Leaflet map initilization.
Better refactoring of leaflet & leaflet_views hooks & apis - Definition of hook_leaflet_formatter_feature_alter
Better marker alt implementation

Initial commit in a new branch, intended to provide a stable, Drupal-9 ready release. Numerous changes since the previous release, so please test and provide as much feedback as possible.