Fix DnD issues regarding data type text (#9347)

* First dragged data should not contain new line character at the beginning
* Return any of "text", "Text" or "text/plain" in the method of getDataTransferText()
* Set only data of type "text" for IE on dragstart
* Clarify return of getDataTransferText() method
* Ignore generated data that is null

client/src/main/java/com/vaadin/client/connectors/grid/GridDragSourceConnector.java

         getDraggedRows(dragStartEvent).forEach(row -> {
             Map<String, String> rowDragData = getRowDragData(row);
             rowDragData.forEach((type, data) -> {
-                if (dataMap.containsKey(type)) {
-                    dataMap.put(type, data);
-                } else {
-                    // Separate data with new line character when multiple rows
-                    // are dragged
-                    dataMap.put(type, dataMap.get(type) + "\n" + data);
+                if (!(data == null || data.isEmpty())) {
+                    if (!dataMap.containsKey(type)) {
+                        dataMap.put(type, data);
+                    } else {
+                        // Separate data with new line character when multiple rows
+                        // are dragged
+                        dataMap.put(type, dataMap.get(type) + "\n" + data);
+                    }
                 }
             });
         });

client/src/main/java/com/vaadin/client/extensions/DragSourceExtensionConnector.java

             // Always set something as the text data, or DnD won't work in FF !
             dataMap.putIfAbsent(DragSourceState.DATA_TYPE_TEXT, "");
 
-            // Set data to the event's data transfer
-            dataMap.forEach((type, data) -> nativeEvent.getDataTransfer()
-                    .setData(type, data));
+            if (!BrowserInfo.get().isIE11()) {
+                // Set data to the event's data transfer
+                dataMap.forEach((type, data) -> nativeEvent.getDataTransfer()
+                        .setData(type, data));
+            } else {
+                // IE11 accepts only data with type "text"
+                nativeEvent.getDataTransfer()
+                        .setData(DragSourceState.DATA_TYPE_TEXT,
+                                dataMap.get(DragSourceState.DATA_TYPE_TEXT));
+            }
 
             // Initiate firing server side dragstart event when there is a
             // DragStartListener attached on the server side

server/src/main/java/com/vaadin/ui/dnd/event/DropEvent.java

     }
 
     /**
-     * Get data of type {@code "text"} from the {@code DataTransfer} object.
+     * Get data of any of the types {@code "text"}, {@code "Text"} or {@code
+     * "text/plain"}.
+     * <p>
+     * IE 11 transfers data dropped from the desktop as {@code "Text"} while
+     * most other browsers transfer textual data as {@code "text/plain"}.
      *
-     * @return Data of type {@code "text"} if exists in the {@code DataTransfer}
-     * object, otherwise {@literal null}.
+     * @return First existing data of types in order {@code "text"}, {@code
+     * "Text"} or {@code "text/plain"}, or {@code null} if none of them exist.
      */
     public String getDataTransferText() {
-        return data.get(DragSourceState.DATA_TYPE_TEXT);
+        // Read data type "text"
+        String text = data.get(DragSourceState.DATA_TYPE_TEXT);
+
+        // IE stores data dragged from the desktop as "Text"
+        if (text == null) {
+            text = data.get(DragSourceState.DATA_TYPE_TEXT_IE);
+        }
+
+        // Browsers may store the key as "text/plain"
+        if (text == null) {
+            text = data.get(DragSourceState.DATA_TYPE_TEXT_PLAIN);
+        }
+
+        return text;
     }
 
     /**

shared/src/main/java/com/vaadin/shared/ui/dnd/DragSourceState.java

      */
     public static final String DATA_TYPE_TEXT = "text";
 
+    /**
+     * Data type {@code "Text"}. IE 11 stores data dragged from the desktop as
+     * "Text" with capital letter.
+     */
+    public static final String DATA_TYPE_TEXT_IE = "Text";
+
+    /**
+     * Data type {@code "text/plain"} for reading data from {@code DataTransfer}
+     * object. Some browsers convert store data with {@code "text"} as {@code
+     * "text/plain"} when transferring data.
+     */
+    public static final String DATA_TYPE_TEXT_PLAIN = "text/plain";
+
     public static final String RESOURCE_DRAG_IMAGE = "drag-image";
 
     /**