Properly communicate forced encodings

Properly communicate forced encodings

This is currently only relevant fo InsetListings, which falls back to
a fixed-width encoding under specific conditions. It is now possible
to query the inset about that and report the correct encoding in
DocIterator::getEncoding.

Addresses the second part of #10995

(cherry picked from commit f924ef29660af5afd22b96744a3e0bb128e1a58d)