CEventLoop: Add wake(), a mechanism for waking up when blocked in select().

This patch generalizes the concept used in Piano to wake up the event loop
so it can react to something happening on a secondary thread.
Basically, there's a pipe who is always part of the file descriptor set we
pass to select(), and calling wake() simply writes a little to that pipe.

Libraries/LibCore/CEventLoop.h

@@ -50,6 +50,8 @@ public:
         m_queued_events.append(move(other.m_queued_events));
     }
 
+    static void wake();
+
 protected:
     virtual void add_file_descriptors_for_select(fd_set&, int& max_fd) { UNUSED_PARAM(max_fd); }
     virtual void process_file_descriptors_after_select(const fd_set&) {}
@@ -69,6 +71,8 @@ private:
     bool m_exit_requested { false };
     int m_exit_code { 0 };
 
+    static int s_wake_pipe_fds[2];
+
     CLock m_lock;
 
     struct EventLoopTimer {