Add split in new window action for editor views.

A few details are still to be done:
- extra windows are not restored yet
- window title of extra windows should show information about current
  editor
- the "raise main window" hack for Linux doesn't work with Qt 5

But it should already now be quite usable.

Change-Id: I638583ee4b2abe32e95c261bdf31b4a10b472748
Reviewed-by: David Schulz <david.schulz@digia.com>

src/plugins/help/externalhelpwindow.cpp

@@ -193,7 +193,7 @@ bool ExternalHelpWindow::eventFilter(QObject *obj, QEvent *event)
             QKeyEvent *keyEvent = static_cast<QKeyEvent*> (event);
             switch (keyEvent->key()) {
                 case Qt::Key_Escape:
-                    Core::ICore::mainWindow()->activateWindow();
+                    Core::ICore::raiseWindow(Core::ICore::mainWindow());
                 default:
                     break;
             }

src/plugins/help/helpplugin.cpp

@@ -729,9 +729,10 @@ void HelpPlugin::showExternalWindow()
     doSetupIfNeeded();
     m_externalWindow->show();
     connectExternalHelpWindow();
-    m_externalWindow->activateWindow();
     if (firstTime)
-        Core::ICore::mainWindow()->activateWindow();
+        Core::ICore::raiseWindow(Core::ICore::mainWindow());
+    else
+        Core::ICore::raiseWindow(m_externalWindow);
 }
 
 void HelpPlugin::modeChanged(Core::IMode *mode, Core::IMode *old)