patch 7.4.1578

Problem:    There is no way to invoke a function later or periodically.
Solution:   Add timer support.

src/structs.h

@@ -2953,3 +2953,18 @@ struct js_reader
     void	*js_cookie;	/* can be used by js_fill */
 };
 typedef struct js_reader js_read_T;
+
+typedef struct timer_S timer_T;
+struct timer_S
+{
+    int		tr_id;
+#ifdef FEAT_TIMERS
+    timer_T	*tr_next;
+    timer_T	*tr_prev;
+    proftime_T	tr_due;		    /* when the callback is to be invoked */
+    int		tr_repeat;	    /* number of times to repeat, -1 forever */
+    long	tr_interval;	    /* only set when it repeats */
+    char_u	*tr_callback;	    /* allocated */
+    partial_T	*tr_partial;
+#endif
+};