Kernel: Add basic process priority support.

For now, the WindowServer process will run with high priority,
while the Finalizer process will run with low priority.
Everyone else gets to be "normal".

At the moment, priority simply determines the size of your time slices.

Kernel/init.cpp

@@ -187,6 +187,7 @@ void init()
     });
     Process::create_kernel_process("Finalizer", [] {
         g_finalizer = current;
+        current->set_priority(Process::LowPriority);
         for (;;) {
             Process::finalize_dying_processes();
             current->block(Process::BlockedLurking);