LibSync: Abstract away Mutex implementation

This commit adds in-place pimpl to abstract away the implementation of
Mutex. It also adds policies to configure the type of mutex desired.

Because of the tight integration between mutex and condition variables
they also needed to be reworked and the changes have to be in one commit
to retain atomicity.

A win32 and pthread implemenation is provided to make sure the api
works with both.

Libraries/LibCore/EventLoopImplementationUnix.cpp

@@ -271,7 +271,7 @@ struct ThreadData {
         s_thread_data.remove(s_thread_id);
     }
 
-    Sync::Mutex mutex;
+    Sync::RecursiveMutex mutex;
 
     // Each thread has its own timers, notifiers and a wake pipe.
     TimeoutSet timeouts;