Set sky API: Add bool for clouds in front of custom skybox

Default true.
Add 'm_clouds_enabled' bool to sky.h, set from new bool in 'set sky' API.
Make 'getCloudsVisible()' depend on 'm_clouds_enabled' instead of
'm_visible' (whether normal sky is visible).

src/client.h

@@ -174,6 +174,7 @@ struct ClientEvent
 			video::SColor *bgcolor;
 			std::string *type;
 			std::vector<std::string> *params;
+			bool clouds;
 		} set_sky;
 		struct{
 			bool do_override;