gui2/tgame_paths, taddon_description: Hide browse buttons if unsupported

Hide the URL/path browse buttons if we don't support the current target
platform, they are wasting space otherwise since they only trigger a
stderr error in that case.

src/gui/dialogs/addon/description.cpp

@@ -393,6 +393,12 @@ void taddon_description::pre_show(CVideo& /*video*/, twindow& window)
 		url_copy_button.set_visible(tcontrol::tvisible::invisible);
 		url_textbox.set_visible(tcontrol::tvisible::invisible);
 	}
+
+	if(!desktop::open_object_is_supported()) {
+		// No point in displaying the button on platforms that can't do
+		// open_object().
+		url_go_button.set_visible(tcontrol::tvisible::invisible);
+	}
 }
 
 } // namespace  gui2

src/gui/dialogs/game_paths.cpp

@@ -105,6 +105,12 @@ void tgame_paths::pre_show(CVideo& /*video*/, twindow& window)
 				boost::bind(&tgame_paths::browse_directory_callback,
 							this,
 							path_path));
+
+		if(!desktop::open_object_is_supported()) {
+			// No point in displaying these on platforms that can't do
+			// open_object().
+			browse_w.set_visible(tcontrol::tvisible::invisible);
+		}
 	}
 }