datetime: Store am_pm_stack in priv structure

This is in preparation for the next patch.

https://bugzilla.gnome.org/show_bug.cgi?id=709443
datetime: Disconnect from GtkStack signals before widget destruction

This fixes a regression from commit 545c886 that caused criticals when
leaving the datetime panel.

https://bugzilla.gnome.org/show_bug.cgi?id=709443
Revert "datetime: Don't leak the dialogs"

This caused criticals when on_permission_changed callback tried to
access priv->datetime_dialog or priv->timezone_dialog before they had
been set up.

Another attempt at fixing the leak in the following patch.

This reverts commit 545c8865434314739d86df6a656a2461e7d1b609.

https://bugzilla.gnome.org/show_bug.cgi?id=709443
datetime: Explicitly destroy toplevel dialogs

https://bugzilla.gnome.org/show_bug.cgi?id=709443
background: Fix reference counting in background XML loader

Commit b9e3603ba4d6b494c0659cbc0196ef9f81fa6a60 added an unref to fix a
memory leak, but it went to a wrong place -- in XML loader, all items
are stored in a hash table that takes ownership of them, and destroyed
when the hash table goes away.

This commit fixes the reference counting in the XML loader and adds
explicit g_object_ref / g_strdup when inserting values to the hash table
to make the memory management more obvious.

The following commit fixes the real leak.

https://bugzilla.gnome.org/show_bug.cgi?id=709453
background: Fix a memory leak with wallpaper loading

This removes an extra g_object_ref when inserting items to GtkListStore:
the list store takes care of incrementing the reference count itself and
the caller doesn't need to do it.

https://bugzilla.gnome.org/show_bug.cgi?id=709453
network: Use the parameters property for 8021x

The argv property was deprecated. This commit converts the 8021x
settings to the parameters property so that the user is able to
setup the 8021x connections.

https://bugzilla.gnome.org/show_bug.cgi?id=709545