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.

Games/Snake/SnakeGame.h

@@ -41,7 +41,7 @@ private:
     explicit SnakeGame(GWidget* parent = nullptr);
     virtual void paint_event(GPaintEvent&) override;
     virtual void keydown_event(GKeyEvent&) override;
-    virtual void timer_event(CTimerEvent&) override;
+    virtual void timer_event(Core::TimerEvent&) override;
 
     struct Coordinate {
         int row { 0 };