I considered replacing this with `gtk_widget_set_opacity`, but it's no
more usable than window manager transparency since the font is transparent.
Since this is already broken with URL hints and won't be around in the
future, it might as well go sooner rather than later.
Closes#128Closes#39
This has been deprecated upstream, and there isn't going to be a way to
preserve the functionality. True transparency is also deprecated as a
feature of the VTE widget, but should be possible with GTK itself.