EditorManager: Do not be a widget.

There is no reason for the editor manager itself to be a widget. This
makes even less sense in the presence of multiple windows.

Change-Id: I384f8945fdd5632d64643e473e6637e05abbce7e
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -189,7 +189,6 @@ MainWindow::MainWindow() :
     m_statusBarManager = new StatusBarManager(this);
     m_messageManager = new MessageManager;
     m_editorManager = new EditorManager(this);
-    m_editorManager->hide();
     m_externalToolManager = new ExternalToolManager();
     setCentralWidget(m_modeStack);