Core: Replace ICoreListener by std::functions

ICoreListener::coreAboutToClose() remains in the core,
ICoreListener::editorAboutToClose() is handled by a new
EditorManager::addCloseEditorListener() function.

This removes the need for some boiler plate code resulting
from the need to implement the interface in custom classes
(DesignModeCoreListener, EditorClosingCoreListener,
PojectEplorer::CoreListener and VcsBase::CoreListener).

EditorManager::addCloseEditorListener

Change-Id: Ie554c987b5455b555be6d77b77e4013639201d22
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/vcsbase/vcsbaseplugin.cpp

@@ -33,7 +33,6 @@
 #include "vcsplugin.h"
 #include "commonvcssettings.h"
 #include "vcsoutputwindow.h"
-#include "corelistener.h"
 #include "vcscommand.h"
 
 #include <coreplugin/documentmanager.h>
@@ -567,7 +566,7 @@ void VcsBasePlugin::initializeVcs(IVersionControl *vc, const Context &context)
     addAutoReleasedObject(vc);
 
     Internal::VcsPlugin *plugin = Internal::VcsPlugin::instance();
-    connect(plugin->coreListener(), &Internal::CoreListener::submitEditorAboutToClose,
+    connect(plugin, &Internal::VcsPlugin::submitEditorAboutToClose,
             this, &VcsBasePlugin::slotSubmitEditorAboutToClose);
     // First time: create new listener
     if (!VcsBasePluginPrivate::m_listener)