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.

Games/Minesweeper/Field.h

@@ -121,7 +121,7 @@ private:
     GButton& m_face_button;
     GLabel& m_flag_label;
     GLabel& m_time_label;
-    RefPtr<CTimer> m_timer;
+    RefPtr<Core::Timer> m_timer;
     int m_time_elapsed { 0 };
     int m_flags_left { 0 };
     Face m_face { Face::Default };