C++ API for MCU

[ogoffart]

With Slint 0.3, we released the platform API for Rust that allows to use the software renderer from a MCU.
But we postponed the work for C++.
This summarize what is left to do.
Some WIP prototype is in the olivier/wip/mcu_cpp branch

Milestone 1: C++ on MCU using a Slint platform written in Rust

• create a main.cpp and CMakeLists.txt file in the printerdemo_mcu folder Created esp-idf component and examples using it.
• make api/cpp no_std and ability to remove all dependencies we don't need
• [ ] add a function to the mcu_board_support crate to initialize the MCU backend. Use it from api/cpp
• add #ifdefs in slint.h for the unsupported stuff on MCU (iostream, thread, and the like)
• Do the right incentation to get CMake generate a static lib
• Serialize the bitmap fonts to C++ datastructure in generator/cpp.rs
• Get cmake to cross compile for such devices

Milestone 2: Platform in C++

• Complete and stabilize the experimental slint_platform.h and slint::experimental::platform namespace. Most of the API in there still relies on raw cbindgen enums for which we need to write a good C++ API