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.
(cherry picked from commit 1c76c77d22)

include/Plugin.h

@@ -124,11 +124,12 @@ 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
 			{
 			}