Add missing hook for on_tt_write in tabbed extension

With this change pty input written with tt_write is properly dispatched
to the active tabbed window when events are sent to the main window.
Patch by Martin Pohlack.

[mina86@mina86.com: pulled from r1.26 in rxvt-unicode CSV repository]
Remove unused variable

[mina86@mina86.com: pulled form r1.28 in rxvt-unicode CSV repository]