AppIcon list moved out of WScreen

The appicon list is moved out of WScreen. The reason is because
the appicon list is used to create and remove icons (appicons, dock,
clip,...) on the screen, but these icons are not associated with the
WScreen. These icons are associated with the Workspace.

If we check the WWorkspace struct we can see that the Clip is inside
this struct. The dock, the background, the workspace name are other
items related to the Workspace, not with the screen.

So, we should take out the appicon from the WScreen. But, what is the
better place to hold it? The workspace? NO!, because the icon list
is common to the all workspaces. Probably the better place is hold
as independent list in the global namespace, so this is my option.

In the next patches we can see that this is the better option, because
we can create the icons, without think where we should paint them.

Signed-off-by: Rodolfo García Peñas (kix) <kix@kix.es>

src/defaults.c

@@ -1216,7 +1216,7 @@ void wReadDefaults(WScreen * scr, WMPropList * new_dict)
 
 void wDefaultUpdateIcons(WScreen *scr)
 {
-	WAppIcon *aicon = scr->app_icon_list;
+	WAppIcon *aicon = w_global.app_icon_list;
 	WDrawerChain *dc;
 	WWindow *wwin = scr->focused_window;