Move EventFilteringMainWindow to utils

Also rename it to AppMainWindow. We add some more functionality to
it in following patches.

Change-Id: I752ccd07759eaaf6a2e96ec758e35b595bf788a2
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -126,7 +126,7 @@ using namespace Core::Internal;
 enum { debugMainWindow = 0 };
 
 MainWindow::MainWindow() :
-    EventFilteringMainWindow(),
+    Utils::AppMainWindow(),
     m_coreImpl(new ICore(this)),
     m_additionalContexts(Constants::C_GLOBAL),
     m_settings(ExtensionSystem::PluginManager::instance()->settings()),