Script: restored compatibility with plugins, creating managed objects

I had to add new PluginObject type of script variable, to distinct it from "normal" DynamicObject.
First, it should not make use of wrapper helpers for reading/writing values, because plugin interface does not declare such functionality, and read and write directly by memory address, as it was done for all objects in vanilla AGS.
Second, member function calls should be handled using vanilla way of calling functions.