Fixed #25004 -- Updated OpenLayers-based widget to OpenLayers 3

Thanks Tim Graham for the review.

django/contrib/gis/forms/widgets.py

@@ -80,13 +80,22 @@ class OpenLayersWidget(BaseGeometryWidget):
     template_name = 'gis/openlayers.html'
 
     class Media:
+        css = {
+            'all': (
+                'https://cdnjs.cloudflare.com/ajax/libs/ol3/3.20.1/ol.css',
+                'gis/css/ol3.css',
+            )
+        }
         js = (
-            'https://cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js',
+            'https://cdnjs.cloudflare.com/ajax/libs/ol3/3.20.1/ol.js',
             'gis/js/OLMapWidget.js',
         )
 
+    def serialize(self, value):
+        return value.json if value else ''
+
 
-class OSMWidget(BaseGeometryWidget):
+class OSMWidget(OpenLayersWidget):
     """
     An OpenLayers/OpenStreetMap-based widget.
     """
@@ -95,12 +104,6 @@ class OSMWidget(BaseGeometryWidget):
     default_lat = 47
     map_srid = 3857
 
-    class Media:
-        js = (
-            'https://cdnjs.cloudflare.com/ajax/libs/openlayers/2.13.1/OpenLayers.js',
-            'gis/js/OLMapWidget.js',
-        )
-
     def __init__(self, attrs=None):
         super(OSMWidget, self).__init__()
         for key in ('default_lon', 'default_lat'):

django/contrib/gis/static/gis/css/ol3.css

@@ -0,0 +1,31 @@
+.switch-type {
+    background-repeat: no-repeat;
+    cursor: pointer;
+    top: 0.5em;
+    width: 22px;
+    height: 20px;
+}
+
+.type-Point {
+    background-image: url("../img/draw_point_off.png");
+    right: 5px;
+}
+.type-Point.type-active {
+    background-image: url("../img/draw_point_on.png");
+}
+
+.type-LineString {
+    background-image: url("../img/draw_line_off.png");
+    right: 30px;
+}
+.type-LineString.type-active {
+    background-image: url("../img/draw_line_on.png");
+}
+
+.type-Polygon {
+    background-image: url("../img/draw_polygon_off.png");
+    right: 55px;
+}
+.type-Polygon.type-active {
+    background-image: url("../img/draw_polygon_on.png");
+}