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.

MQTT: Implements reconnecting in publish thread in case thread is waken-up but client is disconnected.

Note: paho's lostConnection callback got already disabled in previous commit.

MQTT: take C part away from python code for readability.

C lines change a bit because of unnecessary escaping for '\'.

MQTT: WIP, prevent crash after disconnect. Disable auto-reconnect on connection lost callback.

MQTT: Stop publish thread properly on PLC stop

MQTT: Fix overkill use of wx sizer

MQTT: Fix crash when browsing locations

MQTT: WIP. Added handling of received messages (subscriptions)

Dropped perfect hash in favor of bisection
Also fixed indentation (tabs)

MQTT: WIP, added publish thread waking-up when published variables change.

MQTT: WIP, subscibed topics have no "Retained" attribute.

Allow subscribed and published data models to be different, by removing "Retained" column.

MQTT: WIP, now publishes initial values at init.

MQTT: WIP, now connects to broker. Added MQTT 3 support and protocol version selection in IDE.

MQTT: (WIP) Fix OpenSSL linking.

MQTT: still WIP, generated C code builds and link.

Extensions: add optional arguments to ThirdPartyPath

ThirdPartyPath("name", "path1", "path2") will resolve by default into "../name/path1/path2"

GCC BUILD: Prevent Memory Error when compiling huge projects.

Also deleted dead code.

MQTT WIP: intermediate state, added perfect hash to map topic to variables

MQTT WIP: intermediate state, implementing MQTT client C code generator.

WIP: Add skeleton for MQTT extension based on part of OPC-UA extension.

For now generated code makes no sense but persistently-configurable-C-generating-extension infrastructure is there.

Merge remote-tracking branch 'origin/python3' into python3

C runtime: Fix PLCObject.hpp preventing build.

IDE: generalize EVT_GRID_CELL_CHANG{ING->ED} (WxPy4)