Revamp MCU Thermostat example

Revamp Thermostat example so that it works on multiple screen sizes and with regular Qt. Porting to regular Qt has already been done and the example is part of Qt Design Studio examples. This example could be used both for Qt and QUL/MCU - i.e. building a more restricted version from same codebase that runs on MCU (similar complexity as the current thermostat example is) and a version with more functionality when building for Qt.

With larger screens, rooms could be shown in a grid and with smaller ones similarly to current implementation where user can pan horizontally through the list of rooms.

Consider adding xml backend so that the actual and target temperature values would be parsed from xml. Consider if there should be functionality for communicating with the heating system or at least the example should have a placeholder for doing this so that it more closely matches the real use case.

This would be included to the qtdoc Application Examples.

First a UI concept is needed for this.

https://doc.qt.io/QtForMCUs-2.3/quickultralite-thermo-example.html