Added icon feature option to WPrefs: Enforce icon margin

The current findBestIcon function usually selects an icon image
that almost completely fills up the (default) 64x64 pixels of an
icon. As Dan noted in the function, the icon images should use only
75% of the available space, which would result in room for the
miniwindow title and better overall aesthetics.

This feature option provides for enabling such an automatic "icon
shrinking" functionality.

Note: This commit only introduces the new option, not the actual
      image shrinking.

src/WindowMaker.h

@@ -376,6 +376,7 @@ extern struct WPreferences {
 	char disable_root_mouse;           /* disable button events in root window */
 	char auto_focus;                   /* focus window when it's mapped */
 	char *icon_back_file;              /* background image for icons */
+	char enforce_icon_margin;          /* auto-shrink icon images */
 
 	WCoord *root_menu_pos;             /* initial position of the root menu*/
 	WCoord *app_menu_pos;