xC: make terminal attributes abstract

And translate them for frontends.

This is very long overdue, and a rather significant cleanup.

Bump liberty.

NEWS

@@ -1,4 +1,4 @@
-Unreleased
+2.0.0 (Unreleased)
 
  * xD: implemented WALLOPS, choosing to make it target even non-operators
 
@@ -8,6 +8,10 @@ Unreleased
    with the exception of editor_command/editor, backlog_helper/pager,
    and backlog_helper_strip_formatting/pager_strip_formatting
 
+ * xC: all attributes.* configuration options have been made abstract in
+   a subset of the git-config(1) format, and renamed to theme.*,
+   with the exception of attributes.reset, which has no replacement
+
  * xC: replaced behaviour.save_on_quit with general.autosave
 
  * xC: improved pager integration capabilities