core: use a class derived from QList<int> instead of a QList<int> for Core::Context

A mostly mechanical change.

Reviewed-By: con

src/plugins/imageviewer/imageviewer.h

@@ -52,7 +52,7 @@ public:
     explicit ImageViewer(QWidget *parent = 0);
     ~ImageViewer();
 
-    QList<int> context() const;
+    Core::Context context() const;
     QWidget *widget();
 
     bool createNew(const QString &contents = QString());