since qaptworker is dbus spwaned it has a minimal env, in fact it doesn't
even have a locale. to that extent transactions offer a way for the client
to set a locale on the transaction such that the apt output would be
localized and properly encoded.
with debfiles however we decode the file path and pass it to dpkg for
installation which entirely bypassses apt.

two problems are caused by this:
a) the locale is not set up properly as the transaction doesn't use the
   same code paths as a regular apt transaction would
b) qlocale & qtextcodec are constructed way before we work on the
   transaction such that we then decode paths with locale C and latin1
   decoder which results in bogus path decoding

to resolve this the debfile code path will now correctly call the function
used to set up the transaction for apt interaction. this has the
additional boon of making debconf integration actually work.
and additionally this very code path will now explicitly set
a qtextcodec based on what it could parse from the transaction locale.
the result of which should be that deb files from !latin1 paths now works
as expected.

long term both changs are nothing more than workarounds, the transaction
setup bits should be moved somewhere so that it can be called by both
apt bound transactions and debfile transactions (i.e. remove the tie to
apt, even though effectively there is none anyway, so mostly refactor).

for the textcodec workaround two new properties need to be added to the
dbus interfaces, passing QLocale::name and QTextCodec::name such that we
can correctly reconstruct the localization envrionment in the worker.