C++ Logging Hook

[ogoffart]

We should expose a hook in C++ to intercept the panics and logs.

One step will be to use the log crate in Rust for our own logs: #3099
But regardless, we should allow a hook in C++

Proposed API:

namespace slint {
     struct LogMessage {
             enum class Level {
                 /// A panic occurred. This is a bug in Slint. The process is going to be terminated after this.
                 Panic,
                 /// Some recoverable error happened, such as an image could not be loaded
                 Error,
                 //Warn, // (we don't currently have such message, do we?)
                 /// Some information message (eg, the fps from SLINT_DEBUG_PERFORMANCE)
                 Info,
                 /// Output of `debug()` function from slint
                 Debug,
             };
             Level level;
             string_view message;
             string_view file;
             int line;             

             // prevent the user to construct as aggregate that would prevent us to add fields in the future
             private:void*reserved;
     };

     /// Set a hook that is called when Slint would usually display a log message.
     /// If the hook return true, this would mean the default message should not be called.
     void set_message_hook(bool (*) (LogMessage message) );
}

For the panic, the use of string_view message means that one must render the panic in a String, which allocates. And allocating in a panic handler might not be ideal. Alternative is to call the hook for each part of the message, but that's a terrible API imho while allocating in the panic handler should still be rather safe.