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

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.

Update current translations, and add new languages:

- es_MX: Spanish (Mexico)
- pam: Kapampangan

Also update translation process for autoconf-based build system.

Also clarify translation process that the package `gettext` is needed to run string extractor.

- No core strings changed since last time
- Re-add line numbers in .ts file as this is marginally easier for translators (see #1227)

- add part for handling of plurals
- update Transifex links
- small misc changes