When the user replies with a quote to a second message, the composer
moves into a paned below the conversation viewer.  This makes it easy to
scroll through the conversation and select text for replies.

The Gtk.Paned acutally holds a Box, which in turn can hold many
ComposerBoxes.  Only one is shown at a time, but the model used
elsewhere is that each ComposerWidget has a ComposerContainer until it
is destroyed.  When a composer is closed, it hides while finishing up
asynchronous work.  This allows us to hold hidden paned composers as
they finish up their work.

The logic for focus handling at detachment is moved into the
ComposerWidget from ComposerEmbed, since it may also be detached from
the paned state.  ComposerContainers gain a remove_composer() method
that does the container's clean up, as well as returning the focused
widget.  The ComposerWindow's remove_composer() method should never be
called.

https://bugzilla.gnome.org/show_bug.cgi?id=738188