add /statusbar tabmode actlist

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>

src/config/preferences.h

@@ -186,6 +186,7 @@ typedef enum {
     PREF_STROPHE_SM_ENABLED,
     PREF_STROPHE_SM_RESEND,
     PREF_VCARD_PHOTO_CMD,
+    PREF_STATUSBAR_TABMODE,
 } preference_t;
 
 typedef struct prof_alias_t