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/database.c

@@ -28,7 +28,7 @@
 #include "utils.h"
 #include "dbUtils.h"
 #include "update.h"
-#include "main_notify.h"
+#include "event_pipe.h"
 #include "config.h"
 
 #include <glib.h>