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/GAboutDialog.cpp

@@ -30,7 +30,7 @@
 #include <LibGUI/GLabel.h>
 #include <LibGUI/GWidget.h>
 
-GAboutDialog::GAboutDialog(const StringView& name, const GraphicsBitmap* icon, CObject* parent)
+GAboutDialog::GAboutDialog(const StringView& name, const GraphicsBitmap* icon, Core::Object* parent)
     : GDialog(parent)
     , m_name(name)
     , m_icon(icon)