Change string source location in translation files

[rffontenelle]

This changes generate-pot-files.sh to run the commands from the repo root directory, instead from po/.

Note that mediawriter.ts was normally generated inside po/ directory, but this would still make lconvert-qt6 generate the problematic "../src" source location prefix. Therefore I moved mediawriter.ts to root directory and made it a git-ignored to avoid staging it but accident.

Fixes #648

[rffontenelle]

Nevermind. I learned that Weblate can be configured to ignore to leading parent directory ...