MQTT: add JSON payload corresponding to IEC Structured types

- uses https://github.com/cesanta/frozen
- frozen.c is added as a resulting c file in generat_C (fix toolchain_gcc.py as it was breaking dependency checking)
- structured types are converted to C #define, in turn generating json_scanf and json_printf statements

SVGHMI
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Contents:

.. toctree::
   :maxdepth: 2

   intro
   syntax
   pages
   clones
   groups
   variables
   configuration
   i18n
   fonts
   hacking
   widgets
   library