Add helper function to run code on the main UI thread

[mappu]

The example port in #70 uncovered an issue: that MIQT is currently missing functions that can transport a callback onto the main thread.

Draft plan

Possible ideas:

1. moveToThread
2. QCoreApplication::postEvent
3. Emits are allowed cross-thread ⭐

Add cpp:

#include <QObject>

class signaller : public QObject {
	Q_OBJECT
	
private:
	
	do_self_emit_blocking(handle) {
		emit please_run_blocking(handle);
	}
	do_self_emit_async(handle) {
		emit please_run_async(handle);
	}
	
signals:

	void please_run_blocking(handle);
	void please_run_async(handle);
	
public:

	static signaller* singleton() {
		static signaller instance;
		return &instance;
	}
	
	Qt::HANDLE main_thread_id;
	
	static void do_on_main_thread_async(handle) {
		// Always async, no need to run the callback ourselves
		singleton()->do_self_emit_async(handle)
	}
	
	static void do_on_main_thread_blocking(handle) {
		if (QThread::currentThreadId() == main_thread_id) {
			// Call it ourselves to avoid deadlock
			miqt_exec_callback(handle);
		} else {
			singleton()->do_self_emit_blocking(handle)
		}
	}

}

n.b. needs pre-MOC:

• /usr/lib/qt5/bin/moc --help
• /usr/lib/qt6/libexec/moc --help

CABI: After constructing QMainApplication:

// signal comes from another thread, so use queuedconnection
signaller::singleton()->main_thread_id = QThread::currentThreadId();

connect(signaller::singleton(), please_run_blocking, this, [=](handle) {
	miqt_exec_callback(handle);
}, Qt::BlockingQueuedConnection);

connect(signaller::singleton(), please_run_async, this, [=](handle) {
	miqt_exec_callback(handle);
}, Qt::QueuedConnection);

The Go interface:

qt.MainThread(func() {
})

qt.MainThreadAsync(func() {
})

// implementation will call CABI signaler_do_on_main_thread(handle)