LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Applications/SoundPlayer/PlaybackManager.cpp

@@ -29,7 +29,7 @@
 PlaybackManager::PlaybackManager(NonnullRefPtr<AClientConnection> connection)
     : m_connection(connection)
 {
-    m_timer = CTimer::construct(100, [&]() {
+    m_timer = Core::Timer::construct(100, [&]() {
         if (!m_loader)
             return;
         next_buffer();