[JS] Allow storing global leaflet map instances (makinacorpus#204)

This patch allows to store global references to leaflet map instances
created in loadmap() in order to enable more complex customisations
that require direct access to the leaflet map instances

Author: nemesifier

leaflet/templates/leaflet/_leaflet_map.html

@@ -7,8 +7,11 @@
     function loadmap() {
         var djoptions = {% autoescape off %}{{ djoptions }}{% endautoescape %},
             options = {djoptions: djoptions, initfunc: loadmap,
-                       globals: {{ NO_GLOBALS|yesno:"false,true"}}, callback: {{ callback|default:"null" }}};
-        L.Map.djangoMap('{{ name }}', options);
+                       globals: {{ NO_GLOBALS|yesno:"false,true"}}, callback: {{ callback|default:"null" }}},
+            map = L.Map.djangoMap('{{ name }}', options);
+        {% if not NO_GLOBALS %}
+        window['leafletmap' + '{{ name }}'] = map;
+        {% endif %}
     }
     var loadevents = {% autoescape off %}{{ loadevents }}{% endautoescape %};
     if (loadevents.length === 0) loadmap();