feat(demo): add new demo for marker's drag end event

Author: paodb

src/test/java/com/flowingcode/vaadin/addons/googlemaps/DraggableMarkerDemo.java

@@ -0,0 +1,37 @@
+package com.flowingcode.vaadin.addons.googlemaps;
+
+import com.flowingcode.vaadin.addons.demo.DemoSource;
+import com.flowingcode.vaadin.addons.googlemaps.GoogleMap.MapType;
+import com.vaadin.flow.component.html.H2;
+import com.vaadin.flow.component.notification.Notification;
+import com.vaadin.flow.component.orderedlayout.VerticalLayout;
+import com.vaadin.flow.router.PageTitle;
+
+@PageTitle("Draggable Marker Demo")
+@DemoSource(
+    "https://github.com/FlowingCode/GoogleMapsAddon/blob/master/src/test/java/com/flowingcode/vaadin/addons/googlemaps/DraggableMarkerDemo.java")
+@SuppressWarnings("serial")
+public class DraggableMarkerDemo extends VerticalLayout {
+  
+  public DraggableMarkerDemo() {
+    this.setSizeFull();
+    String apiKey = System.getProperty("google.maps.api");
+    if (apiKey == null) {
+      add(
+          new H2(
+              "Api key is needded to run the demo, pass it using the following system property: '-Dgoogle.maps.api=<your-api-key>'"));
+    } else {
+      GoogleMap gmaps = new GoogleMap(apiKey, null, null);
+      gmaps.setMapType(MapType.ROADMAP);
+      gmaps.setSizeFull();
+      gmaps.setCenter(new LatLon(-31.636036, -60.7055271));
+      
+      gmaps.addMarker("Center", gmaps.getCenter(), true, Markers.PURPLE)
+      .addDragEndEventListener(
+          e -> 
+            Notification.show("Lat: " + e.getLatitude() + " - Lng: " + e.getLongitude()));
+                       
+      add(gmaps);
+    }
+  }
+}

src/test/java/com/flowingcode/vaadin/addons/googlemaps/GooglemapsDemoView.java

@@ -34,6 +34,7 @@ public GooglemapsDemoView() {
     addDemo(new AddMarkersDemo());
     addDemo(new AddPolygonsDemo());
     addDemo(new GeolocationDemo());
+    addDemo(new DraggableMarkerDemo());
     setSizeFull();
   }
 }