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/icon.h

@@ -28,6 +28,7 @@
 
 #define TILE_NORMAL	0
 #define TILE_CLIP	1
+#define TILE_DRAWER	2
 
 typedef struct WIcon {
 	WCoreWindow 	*core;