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/LibDraw/SystemTheme.cpp

@@ -51,7 +51,7 @@ void set_system_theme(SharedBuffer& buffer)
 
 RefPtr<SharedBuffer> load_system_theme(const String& path)
 {
-    auto file = CConfigFile::open(path);
+    auto file = Core::ConfigFile::open(path);
     auto buffer = SharedBuffer::create_with_size(sizeof(SystemTheme));
 
     auto* data = (SystemTheme*)buffer->data();