Extend python<->PLC global variable description "GlobalDesc" to support Initial value setting.

Rebased from dporopat change "add initial value to runtime.py file", which means :
- GlobalDesc is used by some customized beremiz python runtime
- Those runtimes need to know what is initial value of these special variables

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<?xml version='1.0' encoding='utf-8'?>

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<BeremizRoot URI_location="LOCAL://">

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  <TargetType/>

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</BeremizRoot>