background: Fix memory leak in the picture sources

The list store was taking a reference to the background item,
so we need to unref it on our side to avoid leaking it.

https://bugzilla.gnome.org/show_bug.cgi?id=709243
background: Fix leak of the preview picture

https://bugzilla.gnome.org/show_bug.cgi?id=709243
background: Fix leak of the Flickr source

The Flickr source was never disposed of when destroying the
selection dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=709243
background: Add test program for the selection dialog

https://bugzilla.gnome.org/show_bug.cgi?id=709243
background: Fix memory leak in Pictures source

The enumerator was never unref'ed.

https://bugzilla.gnome.org/show_bug.cgi?id=709243
background: Fix memory leak in background item object

Some fields were never freed.

https://bugzilla.gnome.org/show_bug.cgi?id=709243
background: Fix memory leaks in slideshow handling

A string was leaked when loading the name of the slideshow,
and a file object was left lingering as well.

https://bugzilla.gnome.org/show_bug.cgi?id=709243
background: Fix memory leak in XML signal handling

When emitted in an idle, the item was correctly unref'ed, but not
when emitting the signal straight away.

https://bugzilla.gnome.org/show_bug.cgi?id=709243