Move timer tick handling into Scheduler.

Kernel/Scheduler.h

@@ -1,20 +1,24 @@
 #pragma once
 
 #include <AK/Assertions.h>
+
 class Process;
+struct RegisterDump;
 
 extern Process* current;
 
 class Scheduler {
 public:
     static void initialize();
+    static void timer_tick(RegisterDump&);
     static bool pick_next();
     static void pick_next_and_switch_now();
     static void switch_now();
     static bool yield();
     static bool context_switch(Process&);
-    static void prepare_for_iret_to_new_process();
     static void prepare_to_modify_tss(Process&);
+private:
+    static void prepare_for_iret_to_new_process();
 };
 
 int sched_yield();