Adwaita: avoid border clash on transitioning adiacent flat buttons

Adwaita: avoid border clash on transitioning adiacent flat buttons

Flat buttons gets the button decoration on hover, while transitioning
the decorations of adiacent flat buttons are both shown (one fading in
and the other fading out) so the borders clashes, since normally there's
no spacing between them, to avoid it the transition on the normal state
is set to none and added back to the hover state, so the decoration
won't fade out. To make the transition more evident the duration is
increased.