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/LibGUI/GMenu.h

@@ -35,7 +35,7 @@
 class GAction;
 class Point;
 
-class GMenu final : public CObject {
+class GMenu final : public Core::Object {
     C_OBJECT(GMenu)
 public:
     explicit GMenu(const StringView& name = "");