Use Common::Flag and Common::Event when possible

Replaces old and simple usages of std::atomic<bool> with Common::Flag
(which was introduced after the initial usage), so it's clear that
the variable is a flag and because Common::Flag is well tested.

This also replaces the ready logic in WiimoteReal with Common::Event
since it was basically just unnecessarily reimplementing Common::Event.

Source/Core/VideoCommon/AsyncRequests.cpp

@@ -19,7 +19,7 @@ AsyncRequests::AsyncRequests() : m_enable(false), m_passthrough(true)
 void AsyncRequests::PullEventsInternal()
 {
   std::unique_lock<std::mutex> lock(m_mutex);
-  m_empty.store(true);
+  m_empty.Set();
 
   while (!m_queue.empty())
   {
@@ -76,7 +76,7 @@ void AsyncRequests::PushEvent(const AsyncRequests::Event& event, bool blocking)
     return;
   }
 
-  m_empty.store(false);
+  m_empty.Clear();
   m_wake_me_up_again |= blocking;
 
   if (!m_enable)