- refactored the thinker list.

This was an unorganized set of static members in DThinker but has now been regrouped into something more structured.

src/dobjgc.cpp

@@ -112,8 +112,6 @@
 
 // EXTERNAL DATA DECLARATIONS ----------------------------------------------
 
-extern DThinker *NextToThink;
-
 // PUBLIC DATA DEFINITIONS -------------------------------------------------
 
 namespace GC
@@ -282,7 +280,6 @@ static void MarkRoot()
 	Mark(StatusBar);
 	M_MarkMenus();
 	Mark(DIntermissionController::CurrentIntermission);
-	DThinker::MarkRoots();
 	Mark(E_FirstEventHandler);
 	Mark(E_LastEventHandler);
 	for (auto Level : AllLevels())
@@ -300,8 +297,6 @@ static void MarkRoot()
 	{
 		Level->Mark();
 	}
-	// NextToThink must not be freed while thinkers are ticking.
-	Mark(NextToThink);
 	// Mark soft roots.
 	if (SoftRoots != NULL)
 	{