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