* gdl/gdl-dock-item.c, gdl/gdl-dock-placeholder.c:
	Added width and height properties for placeholer
	and restore the dock item dimension when they are attached to
	the placeholder. Set placeholder width/height when items are
	hidden.	Prevent placeholders to lose their hosts when the dock
	is reduced to the the root.

	* gdl/gdl-dock-notebook.c: Set the newly docked widget to
	current	page.