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.

Applications/SystemMonitor/ProcessStacksWidget.h

@@ -29,7 +29,9 @@
 #include <LibGUI/GTextEditor.h>
 #include <LibGUI/GWidget.h>
 
-class CTimer;
+namespace Core {
+class Timer;
+}
 
 class ProcessStacksWidget final : public GWidget {
     C_OBJECT(ProcessStacksWidget)
@@ -43,5 +45,5 @@ public:
 private:
     pid_t m_pid { -1 };
     RefPtr<GTextEditor> m_stacks_editor;
-    RefPtr<CTimer> m_timer;
+    RefPtr<Core::Timer> m_timer;
 };