autotools: rearrange qt sources to make them more flexible

Add BITCOIN_MM QR_CPP and BITCOIN_CPP in order to better accomodate complicated
targets. This is a no-op change.
autotools: teach extract_strings_qt.py to respect the XGETTEXT env variable
autotools: add translate target for qt translations
autotools: check for lupdate/xgettext
autotools: update translation docs
Merge pull request #2996 from theuni/qt-translation

Update translation process for autotools