GtkScaleButton: fix unwanted state propagation
GtkScaleButton/GtkVolumeButton use a popover to show content. With the new GTK4 event model clicking or hovering on that child popover produces ACTIVE and PRELIGHT states that are propagated to the main button, this is unwanted behaviour as the button's appearance changes according to those.
We connect to the "state-flags-changed" signal of GtkScaleButton, and remove the unwanted state flags after verifying they're coming from the popover.
Closes issue #2063 (closed)