get_default_icon_filename rewritten

get_default_icon_filename rewritten

The function get_default_icon_filename(), now get_icon_filename(),
was working in a bad way. Before this patch, the function always
searched the default icon in the second search:

-     file_name = wDefaultGetIconFile(winstance, wclass, True);
+     file_name = wDefaultGetIconFile(winstance, wclass, False);

For this reason, the argument default_icon didn't work.

This patch change it. Now, if the default_icon is false, the function
can return NULL, then other functions can do the correct work. For
example, now wDefaultGetImage() doesn't need do nothing, because the
default_icon is set to True. get_icon_filename() checks if the
icon exists in the disk before returning it (except for default icon
in get_default_image_path(scr).