Changed return type of :ICore::mainWindow() to QWidget*

Also removed all <QMainWindow> includes which are not needed
anymore.

Change-Id: I393c9a62a5c6df95d9f35d872e1473a4f13bcdc4
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/coreplugin/icore.cpp

@@ -513,7 +513,7 @@ IContext *ICore::currentContextObject()
 }
 
 
-QMainWindow *ICore::mainWindow()
+QWidget *ICore::mainWindow()
 {
     return m_mainwindow;
 }