components/to_utf8: add class Utf8Encoder
mwiniimporter: use Utf8Encoder
components/translation: use Utf8Encoder
ESMReader, ESMWriter: use Utf8Encoder
components/to_utf8: keep only Utf8Encoder
Add a test for to_utf8 component
Fix to_utf8 test: add test data directory and remove unused include
to_utf8, Utf8Encoder: pass encoding as constructor parameter

Edit other files accordingly.
to_utf8 test: fix Utf8Encoder constructor
Merge remote-tracking branch 'potatoesmaster/to_utf8-rewrite'
clang build fix
Merge remote-tracking branch 'corristo/master'
Merge remote-tracking branch 'upstream/master'