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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These classes are intended for use server-side.

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e.g. in a django view.py :

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    from pyjs.jsonrpc.django import JSONService, jsonremote

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    jsonservice = JSONRPCService()

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    @jsonremote(jsonservice)

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    def test(request, echo_param):

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         return "echoing the param back: %s" % echo_param

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