Add a message tray that slides out when you move your mouse to
the bottom of the screen. The icons for ongoing conversations
are added to the message tray when the first message in the
conversation is received. The icon is removed when the corresponding
conversation window is closed.
Store the avatar icons in the texture cache and use the checksum for
the data bytes for the icon as the key. This allows to reuse the icon
data for the message tray icon.
Add st_box_layout_insert_actor() that allows inserting an actor at the
arbitrary position in the container. It is needed to be able to add the
icon representing the most recent conversation to the front of the list
of icons in the message tray.
