Book desktop should be in the opened books palette

[deyan]

Book desktop is not currently available in opened books palette, it may be shown there as "Show Desktop" for example

[mitex]

Design:
OpenBooksPalette.items: Instead of tracking, filtering and transforming list, use composing list that adds all open books and the desktop view. Use caching of already created items.
In BookItem.render() return "Show Desktop" if the item represents the desktop book.

Implementation:
[6872]

[mira]

* The code for adding an BookItem was not reused for desctop book but copied. I extracted a common method for this.
* You should think of better way for caching BookItems (perhaps using an immutable for instance with good equals() and hashcode() methods)

merged at revision - [6972]