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/GColorPicker.h

@@ -38,7 +38,7 @@ public:
     Color color() const { return m_color; }
 
 private:
-    explicit GColorPicker(Color, CObject* parent = nullptr);
+    explicit GColorPicker(Color, Core::Object* parent = nullptr);
 
     void build();