Send Unicode directional isolate characters around horizontal pane

Send Unicode directional isolate characters around horizontal pane
borders if the terminal support UTF-8 and an extension terminfo(5)
capability "Bidi" is present. On terminals with BiDi support (ie, VTE)
this seems to be enough to display right-to-left text acceptably enough
to be usable (with some caveats about the mouse position). Requested by
and with help from Mahmoud Elagdar in GitHub issue 2425.