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/coreplugin/coreplugin.cpp

@@ -122,7 +122,7 @@ QObject *CorePlugin::remoteCommand(const QStringList & /* options */, const QStr
 {
     IDocument *res = m_mainWindow->openFiles(
                 args, ICore::OpenFilesFlags(ICore::SwitchMode | ICore::CanContainLineNumbers));
-    m_mainWindow->activateWindow();
+    m_mainWindow->raiseWindow();
     return res;
 }