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.

Libraries/LibCore/CEvent.cpp

@@ -27,13 +27,17 @@
 #include <LibCore/CEvent.h>
 #include <LibCore/CObject.h>
 
-CChildEvent::CChildEvent(Type type, CObject& child, CObject* insertion_before_child)
-    : CEvent(type)
+namespace Core {
+
+ChildEvent::ChildEvent(Type type, Object& child, Object* insertion_before_child)
+    : Core::Event(type)
     , m_child(child.make_weak_ptr())
     , m_insertion_before_child(insertion_before_child ? insertion_before_child->make_weak_ptr() : nullptr)
 {
 }
 
-CChildEvent::~CChildEvent()
+ChildEvent::~ChildEvent()
 {
 }
+
+}