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
Show content bundles on home screen

Implements http://wiki.sugarlabs.org/go/Features/Content_support
Merge pull request #43 from walterbender/multi-home-view-4

Add support for multiple homeviews
Remove check for isinstance(activity_info, ActivityBundle)

We merged two pull requests at almost the same time.  Commit 2ffa1c
added this check to _add_activity() in favoritesview.py:

    if not isinstance(activity_info, ActivityBundle):
        return

And commit 2b7766 removed the ActivityBundle import, together with all
those checks.

This commit removes the pending check.
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use get_name(), get_description() instead of name, description in order to circumvent problem with byte-array conversion to string
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