patch 8.0.1278: GUI window always resizes when adding scrollbar

Problem:    GUI window always resizes when adding/removing a scrollbar,
            toolbar, etc.
Solution:   Add the 'k' flag in 'guioptions' to keep the GUI window size and
            change the number of lines/columns instead. (Ychin, closes #703)
diff --git a/src/option.h b/src/option.h
index 88eadae..f9972f2 100644
--- a/src/option.h
+++ b/src/option.h
@@ -235,7 +235,8 @@
 #define GO_TOOLBAR	'T'		/* add toolbar */
 #define GO_FOOTER	'F'		/* add footer */
 #define GO_VERTICAL	'v'		/* arrange dialog buttons vertically */
-#define GO_ALL		"aAbcefFghilmMprtTv" /* all possible flags for 'go' */
+#define GO_KEEPWINSIZE	'k'		/* keep GUI window size */
+#define GO_ALL		"aAbcefFghilmMprtTvk" /* all possible flags for 'go' */
 
 /* flags for 'comments' option */
 #define COM_NEST	'n'		/* comments strings nest */