Conversion from negative float to unsigned is saturating to 0 in rust
and undefined behavior in C++, we should therefore handle the case
properly
Fixes#8222
* C++: add `notify_` prefix to the Model functions and add docs
The `notify_*` function are used by the model to notify the view.
As opposed to the function without the prefix, which are used to get
notified when a source model change
Fixes#3888
CC #3945 (for the docs about thread safety)
Most of the slint.h file is used for model code, so before doing some
work on the model i wanted to refactor the code in its own file.
Since the model need access to the ComponentHandle, i also had to move
that in its own file
