Split get_pixmap_icon_from_icon_win()

Split get_pixmap_icon_from_icon_win()

The function get_pixmap_icon_from_icon_win(), before this patch,
creates the pixmap icon for dockapps (docks with a mini application
inside). The function did three steps:

1. Create the (r)image
2. Create the pixmap using the image
3. Put the application inside the image

Now these three steps are three functions:

step 1: static void get_rimage_icon_from_icon_win(WIcon *icon);
step 2: static void get_pixmap_icon_from_icon_win(WIcon *icon);
step 3: static void set_dockapp_in_icon(WIcon *icon);

These functions contains the same code, that before the changes.
The original function get_pixmap_icon_from_icon_win() includes the
calls to the new functions get_rimage_icon_from_icon_win() and
set_dockapp_in_icon()