button: Style .keyboard-activating on frameless buttons

The use of the keyboard-activating CSS class for buttons was added
in [1], but the style did not apply to buttons with has-frame=FALSE.

[1] 00923615f4 ("button: Add back visual feedback for keynav", 2021-04-01)

gtk/theme/Default/_common.scss

@@ -544,6 +544,7 @@ button {
         transition: $button_transition;
         transition-duration: 500ms;
       }
+      &.keyboard-activating,
       &:active,
       &:checked {
         @include button(undecorated-active);