Adwaita: Make selector more specific

Adwaita: Make selector more specific

A * selector applies to all widgets, so even GtkBox or GtkGrid - and
most importantly GtkListBoxRow - need to recompute their style because
of the * selector.

By using a more specific one, these common cases aren't affected
anymore.

Fixes slowdowns in gtk3-demo's listbox demo and in gnome-software.