patch 9.0.0288: when 'cmdheight' is zero some messages are not displayed

Problem:    When 'cmdheight' is zero some messages are not displayed.
Solution:   Use a popup notification window.
diff --git a/src/structs.h b/src/structs.h
index fc44d23..091b886 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2569,6 +2569,7 @@
     proftime_T	tr_due;		    // when the callback is to be invoked
     char	tr_firing;	    // when TRUE callback is being called
     char	tr_paused;	    // when TRUE callback is not invoked
+    char	tr_keep;	    // when TRUE keep timer after it fired
     int		tr_repeat;	    // number of times to repeat, -1 forever
     long	tr_interval;	    // msec
     callback_T	tr_callback;
@@ -2605,6 +2606,7 @@
     POPPOS_BOTRIGHT,
     POPPOS_TOPRIGHT,
     POPPOS_CENTER,
+    POPPOS_BOTTOM,	// bottom of popup at bottom of screen
     POPPOS_NONE
 } poppos_T;