Fixed crash when opening ui files in design mode

QML Designer now listens to contextChanged signal instead of
modeChanged and checks that the context is same as that of qmldesigner.

Reviewed-by: con

src/plugins/coreplugin/designmode.cpp

@@ -174,6 +174,11 @@ QStringList DesignMode::registeredMimeTypes() const
     return rc;
 }
 
+/**
+  * Registers a widget to be displayed when an editor with a file specified in
+  * mimeTypes is opened. This also appends the additionalContext in ICore to
+  * the context, specified here.
+  */
 void DesignMode::registerDesignWidget(QWidget *widget,
                                       const QStringList &mimeTypes,
                                       const QList<int> &context,