Window comments cleanup

This patch only add some tabs the window.h (only window struct area).

The main change is that moves this block to switchpanel.c, because
is only used there:

       ((w)->wm_gnustep_attr->flags & GSWindowLevelAttr) && \
       ((w)->wm_gnustep_attr->window_level == WMMainMenuWindowLevel || \
        (w)->wm_gnustep_attr->window_level == WMSubmenuWindowLevel))

The patch also include two comments in the window's image:

- WIcon icon is only used when the window is minimized, else is NULL
- RImage has the window image, used in switchpanel,...

src/switchmenu.c

@@ -38,6 +38,11 @@
 #include "workspace.h"
 #include "framewin.h"
 
+#define IS_GNUSTEP_MENU(w) ((w)->wm_gnustep_attr && \
+	((w)->wm_gnustep_attr->flags & GSWindowLevelAttr) && \
+	((w)->wm_gnustep_attr->window_level == WMMainMenuWindowLevel || \
+	 (w)->wm_gnustep_attr->window_level == WMSubmenuWindowLevel))
+
 /********* Global Variables *******/
 extern WPreferences wPreferences;