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/GInputBox.h

@@ -34,7 +34,7 @@ class GTextEditor;
 class GInputBox : public GDialog {
     C_OBJECT(GInputBox)
 public:
-    explicit GInputBox(const StringView& prompt, const StringView& title, CObject* parent = nullptr);
+    explicit GInputBox(const StringView& prompt, const StringView& title, Core::Object* parent = nullptr);
     virtual ~GInputBox() override;
 
     String text_value() const { return m_text_value; }