Ues the new gtk 3.7.10 opacity handling

This handles the corner case of the terminal + scrollbar transparency
that requires the window to be transparent which in turn made colored
text hard to read.
Updated NEWS
Fixed #27: Separate opening and closing animation

Thanks to Giorgos Retsinas for the pull request, even if I've gone a
different road to do it, because many users asked to be able to control
the animation time.

It was the opportunity to kill two birds with one stone.