Kernel: Add magic key combo (Alt+Shift+F12) to dump scheduler state

Pressing this combo will dump a list of all threads and their state
to the debug console.

This might be useful to figure out why the system is not responding.

Kernel/Scheduler.h

@@ -70,6 +70,7 @@ public:
     static Thread& pull_next_runnable_thread();
     static bool dequeue_runnable_thread(Thread&, bool = false);
     static void queue_runnable_thread(Thread&);
+    static void dump_scheduler_state();
 };
 
 }