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.

IDE: allow structures to be located.

merge MQTT grafted from python2 branch - untested

MQTT: add test projects, to be later included in automated testing.

MQTT: add SSL support

SVGHMI: cosmetic

SVGHMI: Fix watchdog not cleanly restarting browser because not waiting for stop operation to finish before restarting.

MQTT: WIP, re-organized disconnection handling, now keep track of connection status and reconnect accordingly.

This fixes bug with non-reconnecting to broker when having no published variables.

MQTT: link to SSL only when needed.

IDE: Fix "Save" toolbar button and file menu entry not appearing when extension makes changes

MQTT: WIP fix modified status not being set when adding and modifying topics or attributes.

Fix loading of CSV that was not applying model types, and that not checking conformance either.

MQTT: WIP, PLC starts even if MQTT client can't connect broker, and publish+subscribe again when reconnecting.

MQTT: WIP, destroys paho client during failure handling at init and at cleanup. Use mutex to protect retrieve.