GtkStackSwitcher: Don't emit notify::visible-child when setting a stack

GtkStackSwitcher: Don't emit notify::visible-child when setting a stack

Synchronizing the visible child with the switcher's toggle buttons can
lead to GtkButton::clicked being emitted twice. Once for the button
that was active before, and once for the button that we just activated.
This leads to notify::visible-child being called twice and one of
them is with the wrong child.

Let's deal with this in the same way we handle the visible child
changing underneath us.

https://bugzilla.gnome.org/show_bug.cgi?id=749021