wmaker: add window app icon to the window list

This patch is adding the app icon in between the flags icon and
the window name from the window list.
Feature request from https://github.com/window-maker/wmaker/issues/19
It is disabled by default, it needs WindowListAppIcons to be
set to YES manually in the conf file or "Show app icons in window list."
enabled from WPrefs expert panel.

src/WindowMaker.h

@@ -479,6 +479,7 @@ extern struct WPreferences {
 	int hot_corner_delay;		   /* Delay after which the hot corner is triggered */
 	int hot_corner_edge;		   /* Hot corner edge size */
 	char *hot_corner_actions[4];	   /* Action of each corner */
+	char window_list_app_icons;        /* Show app icons in window list */
 
 	struct {
 #ifdef USE_ICCCM_WMREPLACE