Ladybird+LibCore: Use QCoreApplication to drive the main Qt event loop

Using QEventLoop works for everything but it breaks *one* little feature
that we care about: automatically quitting the app when all windows have
been closed.

That only works if you drive the outermost main event loop with a
QCoreApplication instead of a QEventLoop. This is unfortunate, as it
complicates our API a little bit, but I'm sure we can think of a way to
make this nicer someday.

In order for QCoreApplication::exec() to process our own
ThreadEventQueue, we now have a zero-timer that we kick whenever new
events are posted to the thread queue.

Userland/Libraries/LibCore/EventLoop.cpp

@@ -170,4 +170,9 @@ bool EventLoop::was_exit_requested() const
     return m_impl->was_exit_requested();
 }
 
+void EventLoop::did_post_event(Badge<Core::ThreadEventQueue>)
+{
+    m_impl->did_post_event();
+}
+
 }

Userland/Libraries/LibCore/EventLoop.h

@@ -19,6 +19,7 @@
 namespace Core {
 
 class EventLoopImplementation;
+class ThreadEventQueue;
 
 // The event loop enables asynchronous (not parallel or multi-threaded) computing by efficiently handling events from various sources.
 // Event loops are most important for GUI programs, where the various GUI updates and action callbacks run on the EventLoop,
@@ -95,6 +96,9 @@ public:
 
     static Function<NonnullOwnPtr<EventLoopImplementation>()> make_implementation;
 
+    void did_post_event(Badge<ThreadEventQueue>);
+    EventLoopImplementation& impl() { return *m_impl; }
+
 private:
     void wait_for_event(WaitMode);
     Optional<Time> get_next_timer_expiration();

Userland/Libraries/LibCore/EventLoopImplementation.h

@@ -37,6 +37,8 @@ public:
     virtual void register_notifier(Notifier&) = 0;
     virtual void unregister_notifier(Notifier&) = 0;
 
+    virtual void did_post_event() = 0;
+
     // FIXME: These APIs only exist for obscure use-cases inside SerenityOS. Try to get rid of them.
     virtual void unquit() = 0;
     virtual bool was_exit_requested() const = 0;

Userland/Libraries/LibCore/EventLoopImplementationUnix.cpp

@@ -522,4 +522,8 @@ void EventLoopImplementationUnix::unregister_notifier(Notifier& notifier)
     ThreadData::the().notifiers.remove(&notifier);
 }
 
+void EventLoopImplementationUnix::did_post_event()
+{
+}
+
 }

Userland/Libraries/LibCore/EventLoopImplementationUnix.h

@@ -31,6 +31,8 @@ public:
     virtual void register_notifier(Notifier&) override;
     virtual void unregister_notifier(Notifier&) override;
 
+    virtual void did_post_event() override;
+
     virtual void unquit() override;
     virtual bool was_exit_requested() const override;
     virtual void notify_forked_and_in_child() override;

Userland/Libraries/LibCore/ThreadEventQueue.cpp

@@ -62,8 +62,11 @@ ThreadEventQueue::~ThreadEventQueue() = default;
 
 void ThreadEventQueue::post_event(Core::Object& receiver, NonnullOwnPtr<Core::Event> event)
 {
-    Threading::MutexLocker lock(m_private->mutex);
-    m_private->queued_events.empend(receiver, move(event));
+    {
+        Threading::MutexLocker lock(m_private->mutex);
+        m_private->queued_events.empend(receiver, move(event));
+    }
+    Core::EventLoop::current().did_post_event({});
 }
 
 void ThreadEventQueue::add_job(NonnullRefPtr<Promise<NonnullRefPtr<Object>>> promise)