Use the LaTeX internal character representation (LICR) macros
provided by lgrenc.def (since version 0.8 from 2013-05-13)
in lib/unicodesymbols. This fixes the PDF bookmarks (except for the
legacy input encoding iso-8859-7) and solves the problem of a missing
"v" character in Libertine LGR fonts (see lyx-users from 2018-01-29).
The ctest unicodesymbols/008-greek-and-coptic_iso8859-7_pdf2" now fails
(due to #9681). This is not a regression, as it is already
"unreliable" (wrong output, Latin character instead of Greek).
Drop compatibility definition of \~ as perispomeni accent
(that was required with lgrenc.def < 0.8).
0d9708ef04