Add support for multiple homeviews

This patch adds support for multiple home views as per [1]. The number
of home views is determined by the list of VIEW_ICONS found in
/desktop/sugar/desktop/view_icons. If no gconf entry is found,
defaults from jarabe/desktop/config.py are used. The icons in the list
are used both for the home view selection on the Home View toolbar and
the favorites list in the Home List View. Just one home view, using
the view-radial icon is used by default. Similarly, in the list view,
the favorities icons are found in
/desktop/sugar/desktop/favorite_icons. If not favorite icons are
found, emblem-favorites is used.

A unittest is included.

[1] http://wiki.sugarlabs.org/go/Features/Multiple_home_views
new desktop model

(need to complete multi-desktop commit)
new unit test for multi-homeview
adding view used by unit test
add_bundle does not specify a favorite, so a default value must be provided to _add_bundle
Merge pull request #43 from walterbender/multi-home-view-4

Add support for multiple homeviews