Replace deprecated signals

The 'size-request' and 'expose-event' signals now don't exist anymore.
'expose-event' is simply replaced by 'draw'. We used 'size-request' in a
slightly different way than it was supposed to be: connecting a handler
after the default 'size-request' handler run, ensured that the handler
was run after the GtkTextView finished validating text positions, so we
could scroll the text view to the end after printing text to it. Luckily,
'size-allocate' works the same way in GTK 3.