Core: Reduce code path ping-pong between ICore and MainWindow

Public entry points for displaying settings dialog were in ICore,
diverting to MainWindow, and back in some cases.

Move implementation to icore.cpp instead.

Change-Id: I02cbf1dcfe6241c665d7d701b4b4af1a8a242af5
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/mainwindow.h

@@ -111,13 +111,6 @@ public slots:
     void openFileWith();
     void exit();
 
-    bool showOptionsDialog(Id page = Id(), QWidget *parent = 0);
-
-    bool showWarningWithOptions(const QString &title, const QString &text,
-                                const QString &details = QString(),
-                                Id settingsId = Id(),
-                                QWidget *parent = 0);
-
 protected:
     virtual void closeEvent(QCloseEvent *event);