gtkplacesviewrow: stop busy_spinner from offsetting the other widgets when visible

gtkplacesviewrow: stop busy_spinner from offsetting the other widgets when visible

...by putting it in a stack. The busy_spinner and eject_button are
mutually exclusive, but only the latter was coded to ensure that its
visibility did not cause the rest of the row to reflow. By putting both
widgets in a stack and setting child_visible on that, the row allocates
enough space to show one - or none - at once, avoiding any misalignment.

https://bugzilla.gnome.org/show_bug.cgi?id=772345

https://bugzilla.gnome.org/show_bug.cgi?id=772348