Add background image to Homeview

This series of patches adds a background image to the homeview as per
[1]. The patch impacts homebox.py and introduces a new control panel
widget for selecting a background image.

Some details:
(1) Two gconf settings are used to tell homebox.py which image to use for the background and what alpha setting to use when rendering the image
(2) Currently the control panel widget only allows the user to set the background image; a reasonable default alpha setting (75%) is used.
(3) The images available for the background are found in ~/Documents

Some discussion:
There are several suggestions for future enhancements to the patch series, including exposing some mechanim for setting alpha; adding a background to the other zoom levels; and eliminating the need to restart Sugar in order to see the changes.

This series of patches is based on the work of Agustin Zubiaga.

[1] http://wiki.sugarlabs.org/go/Features/Background_image_on_home_view
Merge branch 'background' of https://github.com/walterbender/sugar into background