Keyboard shortcut to raise the dock

This patch adds the DockRaiseLowerKey shortcut, which raises/lowers
the dock depending on whether the dock is lowered/raised.

[crmafra: Reformatted Brad's patch against git repo and removed the
 DockRaiseKey and DockLowerKey shortcuts ]

src/keybind.h

@@ -50,6 +50,9 @@ enum {
 	/* window, menu */
 	WKBD_CLOSE,
 
+	/* Dock */
+	WKBD_DOCKRAISELOWER,
+
 	/* Clip */
 	WKBD_CLIPLOWER,
 	WKBD_CLIPRAISE,