toolbar: Use GTK_STYLE_CLASS'es on the pathbar container box

On wider windows, we show a border around the pathbar and fill it
with background color using custom CSS specific for Adwaita.

This means the pathbar is unbounded in HighContrast mode or 3rd party
custom themes, giving the headerbar an uncentered, unbalanced layout.

Fix that using the "frame" and "background" style class which are
exposed by Gtk+ for this purpose.

Closes: #661
3 jobs for 661-broken-pathbar-w-high-contrast-on-nautilus-3-30-2 in 11 minutes and 18 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed flatpak #115255

00:10:56

 
  Deploy
passed review #115256

00:00:21

manual stop_review #115257
allowed to fail manual