The existing way how active tabs are displayed didn't allow showing more
than 10 tabs. This patch adds a mode where the statusbar shows a
comma-separated list of tabs which were active since the last time viewed.
This view is inspired by how `irssi` shows the active tabs, therefore
it is also called `actlist`.
Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>