Multi identify

[manisandro]

Adds a multiSelection property to the Identify component which allows multiple identify requests to be captured without purging the results if the ctrl modifier is pressed when clicking.

Note: I've been unsuccessful in finding any way to simulate a click in openlayers to implement the 'click' test below... Any ideas how this can be done? The way ol upstream does it[1] in the test suite relies on many private methods...

+    it('check if the handler for "click" event is called', (done) => {
+        const testHandlers = {
+            handler: () => {}
+        };
+        var spy = expect.spyOn(testHandlers, 'handler');
+
+        const map = ReactDOM.render(
+            <OpenlayersMap
+                center={{y: 43.9, x: 10.3}}
+                zoom={11}
+                onClick={testHandlers.handler}
+            />
+        , document.getElementById("map"));
+
+        const olMap = map.map;
+        const mapDiv = olMap.getViewport();
+
+        let downEvent = document.createEvent('MouseEvents');
+        downEvent.initEvent ("mousedown", true, true);
+        mapDiv.dispatchEvent(downEvent);
+        let upEvent = document.createEvent('MouseEvents');
+        upEvent.initEvent ("mouseup", true, true);
+        mapDiv.dispatchEvent(upEvent);
+        let clickEvent = document.createEvent('MouseEvents');
+        clickEvent.initEvent ("click", true, true);
+        mapDiv.dispatchEvent(clickEvent);
+
+//         mapDiv.click();
+        setTimeout(() => {
+          try {
+            expect(spy.calls.length).toEqual(1);
+            expect(spy.calls[0].arguments.length).toEqual(1);
+            expect(spy.calls[0].arguments[0].pixel).toExist();
+            expect(spy.calls[0].arguments[0].latlng).toExist();
+            expect(spy.calls[0].arguments[0].modifiers).toExist();
+            expect(spy.calls[0].arguments[0].modifiers.altKey).toEqual(false);
+            expect(spy.calls[0].arguments[0].modifiers.ctrlKey).toEqual(false);
+            expect(spy.calls[0].arguments[0].modifiers.shiftKey).toEqual(false);
+          } catch(e) {
+            return done(e);
+          }
+            done();
+        }, 600);
+    });

[1] https://github.com/openlayers/ol3/blob/master/test/spec/ol/interaction/modify.test.js#L80

[coveralls]

Coverage increased (+0.1%) to 80.281% when pulling 7c3a0c3 on sourcepole:multi_identify into bdeb615 on geosolutions-it:master.