theme: Use $button_transition on check+radio nodes

This makes the change in the previous commit transition nicely, as with
'normal' buttons.

gtk/theme/Default/_common.scss

@@ -2562,6 +2562,7 @@ radio {
   min-width: 14px;
   border: 1px solid;
   -gtk-icon-source: none;
+  transition: $button_transition;
 
   & {
     // for unchecked
@@ -2570,6 +2571,7 @@ radio {
     @each $state, $t in ("", "normal"),
                         (":hover", "hover"),
                         (":active", "active"),
+                        (":backdrop", "backdrop"),
                         (":disabled", "insensitive") {
       &#{$state} {
         @include check($t, $_c);
@@ -2584,6 +2586,7 @@ radio {
         @each $state, $t in ("", "normal"),
                             (":hover", "hover"),
                             (":active", "active"),
+                            (":backdrop", "backdrop"),
                             (":disabled", "insensitive") {
           &#{$state} {
             @include check($t, $checkradio_bg_color, $checkradio_fg_color, $checked: true);