Turn filter list options into context menu

Let the menu introduced in commit 5e86442bab
pop up on right-click instead of button-click.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/simplewidgets.h

@@ -167,6 +167,7 @@ public:
 };
 
 class FilterBase : public QWidget {
+	void addContextMenuEntry(const QString &s, void (FilterModelBase::*)());
 protected:
 	FilterBase(FilterModelBase *model, QWidget *parent = 0);
 	FilterModelBase *model;