SVGHMI: code cleanup

MQTT: Add status global variable MQTT_STATUS_n, one per MQTT client

MQTT: strip unsupported types from type choice dropdown, better wrong type detection in build.

MQTT: remove debug prints

MQTT: add JSON fixed size 1D array support. WIP. Parsing array still broken.

MQTT: do not convert to UPPERCASE json keys.

MQTT: added support for strings in JSON payload only.

MQTT: remove accidentally committed debug printfs + cosmetic

MQTT: fix invocation of json_scanf (use pointers, not values...)

MQTT: fix bisection failing when topics start with same substring.

MQTT: Update available type choices when data types are changed in the IDE

MQTT: remove standalone MQTT clien generator code.

Code was inherited from OPC-UA extension, but not properly maintained.

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

MQTT: WIP supporting structured types : do not crash when generating C code if using location with no size

MQTT: add dropdown to select type assigned to a topic

MQTT+IDE: Allow user to associate IEC structure datatype to a topic.