ResourceItem: always return name extension in lower case

Always return name extension in lower case so we can easily match it
somewhere else or use it as key for hash tables.

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>

include/ResourceItem.h

@@ -95,7 +95,7 @@ public:
 
 	inline QString nameExtension() const
 	{
-		return name().section( '.', -1 );
+		return name().section( '.', -1 ).toLower();
 	}
 
 	Type type() const