widget: search for an alternative icon if none available

If no icon is available for the tasklist, search for an alternative
one. Also set the application icon to avoid to search again
later. Maybe this would be better to search for an icon when managing
the new window?

rc/widgets.lua

@@ -175,7 +175,26 @@ for s = 1, screen.count() do
     if screen.count() > 1 then
        tasklist[s]  = awful.widget.tasklist(
 	  function(c)
-	     return awful.widget.tasklist.label.currenttags(c, s)
+	     local title, color, _, icon = awful.widget.tasklist.label.currenttags(c, s)
+	     -- Try to search for an alternative icon if none is available
+	     for _, name in pairs({c.class, c.instance}) do
+		if not icon and title and name then
+		   for _, n in pairs({name, name:lower()}) do
+		      icon = awful.util.geticonpath(name,
+						    nil,
+						    {"/usr/share/fvwm-crystal/fvwm/icons/Default/16x16/apps/",
+						     "/usr/share/fvwm-crystal/fvwm/icons/Default/22x22/apps/",
+						     "/usr/share/icons/hicolor/16x16/apps/"})
+		      if icon then
+			 -- This is our new icon. And we set it for the client to not search again
+			 icon = image(icon)
+			 c.icon = icon
+		      end
+		   end
+		end
+	     end
+	     -- Use our icon and don't set the status image.
+	     return title, color, nil, icon
 	  end, tasklist.buttons)
     else
        tasklist[s] = ""