Fix selection of empty editor view

- open QtC and switch to edit mode
- split the editor view
- click on the empty views

The view you clicked on should get selected (color changes and the
colored bar moves to it). This broke when implementing back/forward
gesture support which accidentally stopped propagating events on the
views. Also fix the corresponding code for the help viewer.

Amends 789872a0f9

Change-Id: Iee1a58ac62451d96a2533153edb40a4272dd0b0e
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/coreplugin/editormanager/editorview.cpp

@@ -338,7 +338,7 @@ bool EditorView::event(QEvent *e)
             }
         }
     }
-    return false;
+    return QWidget::event(e);
 }
 
 void EditorView::addEditor(IEditor *editor)

src/plugins/help/helpviewer.cpp

@@ -204,7 +204,7 @@ bool HelpViewer::event(QEvent *e)
             }
         }
     }
-    return false;
+    return QWidget::event(e);
 }
 
 void HelpViewer::incrementZoom(int steps)