Ability to hide the mouse pointer on a keypress a la Word

[originally from svn r883]

putty.h

@@ -196,6 +196,7 @@ typedef struct {
     int fontcharset;
     char logfilename[FILENAME_MAX];
     int logtype;
+    int hide_mouseptr;
     /* Colour options */
     int try_palette;
     int bold_colour;