Replace eprintln with use of the log facade

[tronical]

This allows rust applications to have more control over the output the run-time library generates.

[tronical]

Follow-up question #1:

Should all our Rust examples use env_logger by default perhaps?

[tronical]

For C++ the path forward would be basically a front-end API to allow setting a hook, like what Olivier proposed:

enum LogType {
    SlintDebug, //< The output of `debug()`in slint
    Info, //< (Not sure)
    Warning, // Some non-fatal error happened 
    Error, // Something really bad hapenned
}
using LogHook = void (*) (LogType type, std::string_view message);

slint::set_log_hook([](type, message) {
  switch (type) { 
     //...
  }
 } );

[ogoffart]

does it make sense to keep it as optional then?

And where are the println! going if no log backend is selected?
Are they then going to be invisible. Making it even harder for use to figure out problems?

[ogoffart]

Should all our Rust examples use env_logger by default perhaps?

i'm thinking the slint should enable a logger by default that print to stderr or something like that if the "log" feature is not set.
It is otherwise pretty terrible if out of the box, a simple hello world don't show the warning on what went wrong if something goes wrong.

[ogoffart]

We could have a mod log in i_slint_core, that re-export the macro from the log crate, or just implement macro that print to stderr depending on a feature flag. (or should it be in i_slint_common since it is also used there?)

[ogoffart]

We should also remove the debug_log macro in favor of this.