PathGameObject saveable check now checks for references in all sectors

Fixes an issue, introduced in SuperTux#2320, regarding the `PathGameObject::is_saveable()` function's implementation.

The issue caused paths to not be saveable, if the current sector, opened in the editor, does not have any PathObjects that reference them. The check is now performed for all sectors in a level.

src/object/path_gameobject.cpp

@@ -24,6 +24,7 @@
 #include "sprite/sprite.hpp"
 #include "sprite/sprite_manager.hpp"
 #include "supertux/debug.hpp"
+#include "supertux/level.hpp"
 #include "supertux/sector.hpp"
 #include "util/log.hpp"
 #include "util/reader_mapping.hpp"
@@ -215,11 +216,12 @@ PathGameObject::is_saveable() const
   if (!Sector::current())
     return false;
 
-  const auto& path_objects = Sector::get().get_objects_by_type<PathObject>();
-
-  for (const auto& path_obj : path_objects)
-    if (path_obj.get_path_gameobject() == this)
-      return true;
+  for (const auto& sector : Level::current()->get_sectors())
+  {
+    for (const auto& path_obj : sector->get_objects_by_type<PathObject>())
+      if (path_obj.get_path_gameobject() == this)
+        return true;
+  }
 
   return false;
 }

src/supertux/level.hpp

@@ -52,6 +52,7 @@ class Level final
 
   size_t get_sector_count() const;
   Sector* get_sector(size_t num) const;
+  const std::vector<std::unique_ptr<Sector> >& get_sectors() const { return m_sectors; }
 
   std::string get_tileset() const { return m_tileset; }