Allow external windows to define contexts for Qt Creator.

Task-number: QTCREATORBUG-5070

Change-Id: Ia466b73eb37bb8aef6f71ab8f1d990b40b43cac3
Reviewed-by: Konstantin Tokarev <annulen@yandex.ru>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/mainwindow.cpp

@@ -1122,9 +1122,8 @@ void MainWindow::updateFocusWidget(QWidget *old, QWidget *now)
         return;
 
     IContext *newContext = 0;
-    if (focusWidget()) {
+    if (QWidget *p = qApp->focusWidget()) {
         IContext *context = 0;
-        QWidget *p = focusWidget();
         while (p) {
             context = m_contextWidgets.value(p);
             if (context) {