qt: add transifex configuration file

This is a project-wide configuration file and should be the same for
everyone.
Also remove mention of creating it yourself from the translation process.
devtools: add a script to fetch and postprocess translations

Run this script from the root of the repository to update all translations from transifex.
It will do the following automatically:

- create a transifex configuration file
- fetch all translations
- post-process them into valid and committable format
Full translation update

Last update (48be9ce) missed quite a lot, for some reason.

This is also the first update done with the new script
`contrib/devtools/update-translations.py`
Merge pull request #4110

3a54ad9 Full translation update (Wladimir J. van der Laan)
9dd5d79 devtools: add a script to fetch and postprocess translations (Wladimir J. van der Laan)
58c01a3 qt: add transifex configuration file (Wladimir J. van der Laan)