Add drawers to wmaker!

Drawers are horizontal docks, and they can themselves only live in the dock

To use them, right click on the dock or a docked appicon and select "Add
a drawer". Then move appicons into the drawer (drag them with the
mouse). You may change the icon of the drawer. By default, drawers
auto-expand and -collapse, and auto-raise/lower. This can be customized
in the same way as for the clip.

Set DisableDrawers to YES in G/D/WindowMaker if you do not want to see
the menu entry to add a drawer.

Just discovered this bug: the auto-attract icon functionality will not
work (to be precise, it crashes WM!) if the clip is disabled
(NoClip=YES). Will fix shortly, of course.

src/WindowMaker.h

@@ -433,6 +433,7 @@ typedef struct WPreferences {
     struct {
         unsigned int nodock:1;	       /* don't display the dock */
         unsigned int noclip:1;         /* don't display the clip */
+        unsigned int nodrawer:1;       /* don't use drawers */
         unsigned int noupdates:1;      /* don't require ~/GNUstep (-static) */
         unsigned int noautolaunch:1;   /* don't autolaunch apps */
         unsigned int norestore:1;      /* don't restore session */