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.

Meta/Lagom/TestApp.cpp

@@ -30,9 +30,9 @@
 
 int main(int, char**)
 {
-    CEventLoop event_loop;
+    Core::EventLoop event_loop;
 
-    auto timer = CTimer::construct(100, [&] {
+    auto timer = Core::Timer::construct(100, [&] {
         dbg() << "Timer fired, good-bye! :^)";
         event_loop.quit(0);
     });