Core: Improve dialogParent() to work better with detached windows

Make new windows center on the window the user is currently looking
at instead of the mainwindow.

Change-Id: Ia6246a4f1935b6d2d66db329c6141f4cb8349916
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/icore.cpp

@@ -488,7 +488,11 @@ QWidget *ICore::mainWindow()
 QWidget *ICore::dialogParent()
 {
     QWidget *active = QApplication::activeModalWidget();
-    return active ? active : m_mainwindow;
+    if (!active)
+        active = QApplication::activeWindow();
+    if (!active)
+        active = m_mainwindow;
+    return active;
 }
 
 QStatusBar *ICore::statusBar()