main_notify: renamed source to event_pipe.c

We are going to migrate away from the concept of notifying the main
thread.  There should be events sent to it instead.  This patch starts
a series to implement that.

src/idle.c

@@ -22,7 +22,7 @@
  */
 
 #include "idle.h"
-#include "main_notify.h"
+#include "event_pipe.h"
 
 #include <assert.h>
 #include <glib.h>