Isolate text selection blocking to Grid instead of body element (#16838)

Some themes are more explicit with user-select rules, and doing this on
the level of body element does not work with said themes. To make all
the themes behave the same way, we only block selection inside of Grid.

Change-Id: Iaf24fede88e4bc683e065c214e7baf3f545c99c9

client/src/com/vaadin/client/widgets/Grid.java

@@ -36,7 +36,6 @@ import com.google.gwt.core.client.Scheduler.ScheduledCommand;
 import com.google.gwt.core.shared.GWT;
 import com.google.gwt.dom.client.BrowserEvents;
 import com.google.gwt.dom.client.DivElement;
-import com.google.gwt.dom.client.Document;
 import com.google.gwt.dom.client.Element;
 import com.google.gwt.dom.client.EventTarget;
 import com.google.gwt.dom.client.NativeEvent;
@@ -5581,24 +5580,25 @@ public class Grid<T> extends ResizeComposite implements
                                                     c));
                                         }
                                     }
-                                    WidgetUtil.setTextSelectionEnabled(Document
-                                            .get().getBody(), false);
+
+                                    WidgetUtil.setTextSelectionEnabled(
+                                            getElement(), false);
                                 }
 
                                 @Override
                                 public void onComplete() {
                                     fireEvent(new ColumnResizeEvent<T>(col));
 
-                                    WidgetUtil.setTextSelectionEnabled(Document
-                                            .get().getBody(), true);
+                                    WidgetUtil.setTextSelectionEnabled(
+                                            getElement(), true);
                                 }
 
                                 @Override
                                 public void onCancel() {
                                     col.setWidth(initialWidth);
 
-                                    WidgetUtil.setTextSelectionEnabled(Document
-                                            .get().getBody(), true);
+                                    WidgetUtil.setTextSelectionEnabled(
+                                            getElement(), true);
                                 }
                             });
                     dragger.addTo(td);