patch 9.0.0917: the WinScrolled autocommand event is not enough

Problem:    The WinScrolled autocommand event is not enough.
Solution:   Add WinResized and provide information about what changed.
            (closes #11576)

src/vim.h

@@ -1407,7 +1407,8 @@ enum auto_event
     EVENT_WINCLOSED,		// after closing a window
     EVENT_VIMSUSPEND,		// before Vim is suspended
     EVENT_VIMRESUME,		// after Vim is resumed
-    EVENT_WINSCROLLED,		// after Vim window was scrolled
+    EVENT_WINRESIZED,		// after a window was resized
+    EVENT_WINSCROLLED,		// after a window was scrolled or resized
 
     NUM_EVENTS			// MUST be the last one
 };