Provide API for polling Libcamera-apps message queue

[matswebjorn]

I'm using libcamera-apps-v1.7.0 integrated into a multithread application. I therefore can't use RPiApp::Wait() method to wait for camera request to complete as it would lock up my entire application. The solution (so far) is therefore to add a method

bool RPiCamApp::QueueEmpty() { return msg_queue_.QueueEmpty(); }

which I can use to poll Libcamera for request status. Then in RpiCamApp::MessageQueue

bool QueueEmpty()
{
	std::unique_lock<std::mutex> lock(mutex_);
	return queue_.empty();
}