themes: Fix swapped borders on RTL PathBar buttons

themes: Fix swapped borders on RTL PathBar buttons

.linked assumes the container is a GtkBox, which is documented as never
flipping children in RTL, so :first-child is always the left child, etc.
GtkBox does that by reordering its CSS nodes when the direction changes.

But most widgets don’t do that, so :first|last-child are 1st/last ADDED
and swap sides in RTL. GtkPathBar is so, and ignoring that in our themes
meant that in RTL, its left/right buttons got each other’s borders. Yuk!

This patch adds the groundwork for supporting widgets like that, via the
%linked_flippable placeholder, and applies that to override buttons in
  filechooser .path-bar.linked > button
so that the correct borders get applied to those buttons when using RTL.

Note that I select only PathBars within a FileChooser because we also
have NautilusPathBar, which also uses widget.path-bar – but *does* flip
its nodes for RTL already, so letting that get affected broke it again!

https://bugzilla.gnome.org/show_bug.cgi?id=772817