Returns a handle to the entity whose name equals entityName. Returns null if it couldn't be found.
AddEntity("bowser.ent", pos);
ETHEntity@ bowser = SeekEntity("bowser.ent");
AddEntity("level_boss.ent", pos, "bowser");
ETHEntity@ bowser = SeekEntity("bowser");
If there are multiple entities with matching names, the entity handle returned by SeekEntity will point to the first one it could find.
When the entity found is temporary, this function returns null.