Plugin::Descriptor::SubPluginFeatures: made some methods const

We can safely make fillDescriptionWidget() and listSubPluginKeys()
const methods. Furthermore descriptor pointer to listSubPluginKeys()
can be const as well. Might allow more optimizations by the compiler.

include/Plugin.h

@@ -133,11 +133,12 @@ public:
 			{
 			}
 
-			virtual void fillDescriptionWidget( QWidget *, const Key * )
+			virtual void fillDescriptionWidget( QWidget *, const Key * ) const
 			{
 			}
 
-			virtual void listSubPluginKeys( Plugin::Descriptor *, KeyList & )
+			virtual void listSubPluginKeys( const Plugin::Descriptor *,
+															KeyList & ) const
 			{
 			}