modalDialog: emit 'closed' signal when dialog is closed

modalDialog: emit 'closed' signal when dialog is closed

Opening and closing a modal dialog is not instant. There
is a transition animation involved.  When the dialog is
finished opening, it currently emits the "opened" signal.

When it's finished closing, however, it doesn't emit a
"closed" signal.  This means, there's not a good way to
know when the dialog finishes closing.

This commit adds the "closed" signal.

https://bugzilla.gnome.org/show_bug.cgi?id=694296