Adwaita: use child istead of descendant selectors for linking

Since widgets subelements are now both widget childs and widget
classes, say .spinbutton.button and .spinbutton .button, the linking
logic needs to be adapted to be stricter with child selectors.
This fixes vertically linked spinbuttons.
HC: sidebars and vertical toolbars with borders

- the assumption for vertical toolbar being on the left is perhaps
  a little daring, but...

https://bugzilla.gnome.org/show_bug.cgi?id=740859
HC: use child istead of descendant selectors for linking

Since widgets subelements are now both widget childs and widget
classes, say .spinbutton.button and .spinbutton .button,
the linking logic needs to be adapted to be stricter with
child selectors. This fixes vertically linked spinbuttons.