Fix GTK frame clock warnings with our about dialog

We used to set the widget's style in a style-set handler, and despite
guarding ourselves from recursive invocations, the GTK frame clock
introduced by GTK 3.8 didn't like it very much.

So, replace the manual color modification by a simpler global widget
state modification, making the code simpler and smaller, yet achieving
the same result (or so I hope and see).