Fix issues in the entity creation and destruction code.

Implement a locking mechanism to prevent entities be dstroyed in mid-destruction.
Also clean up properly when internal instances are deleted.

source/SqBase.hpp

@@ -385,7 +385,8 @@ enum EventType
 enum EntityFlags
 {
     ENF_DEFAULT     = (0),
-    ENF_OWNED       = (1 < 0)
+    ENF_OWNED       = (1 << 1),
+    ENF_LOCKED      = (2 << 1)
 };
 
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