Create attribute "leaflet_id" for LayerGroups

When a LayerGroup is created, then it gets a numerical id. Thus the layer containing all markers (for example) is not easily accessible in client-side Javascript.

A simple solution is to add the attribute 'leaflet_id' to the group feature when creating the the data structure of the map.

$features_group = array(
    'leaflet_id' => 'markers',
    'group' => 'markers',
    'label' => t('Markers'),
    'features' => $features,
);

In leaflet.drupal.js this attribute is recognized when creating the LayerGroup and the LayerGroup can be accessed, like in this example:

var markerLayer = Drupal.settings.leaflet[0].lMap._layers['markers']