ci: Add potfile check

This script checks for files that are translatable, but are missing from POTFILES.in. Also, it checks whether files that are in POTFILES.in are translatable at all or are missing, as well as the sorting of POTFILES.in.

Based on nautilus!1418 (merged).

Closes #2845 (closed)

The output before the changes to POTFILES.in:

The following files are missing from po/POTFILES.in or po/POTFILES.skip:

  panels/printers/pp-new-printer.c
  panels/system/users/data/gnome-user-accounts-panel.desktop.in

The following files are in po/POTFILES.in but are missing or not translatable:

  panels/background/cc-background-chooser.ui
  panels/background/cc-background-preview.ui
  panels/common/cc-language-chooser.c
  panels/common/cc-time-entry.c
  panels/keyboard/cc-keyboard-item.c
  panels/keyboard/cc-xkb-modifier-dialog.ui
  panels/mouse/cc-mouse-test.c
  panels/network/net-vpn.c
  panels/online-accounts/cc-online-accounts-panel.c
  panels/power/cc-power-profile-row.ui
  panels/printers/pp-samba.c
  panels/sharing/cc-sharing-networks.ui
  panels/system/region/cc-region-page.c
  panels/system/users/cc-users-page.c
  panels/universal-access/cc-ua-panel.c
  panels/wacom/cc-wacom-panel.c
  panels/wacom/cc-wacom-stylus-action-dialog.ui

The following file is not sorted properly in po/POTFILES.in:

  panels/privacy/cc-location-page.c

cc @felipeborges

This needs to be added to CI as well, but I first wanted to share the script. This also adds a check for files that are in POTFILES.in but are missing or not translatable, which might be handy as well?

CI changes
I have committed it to the CI now, it passes at the moment with the changes to POTFILES.in. So all MRs would at least have to rebase before running the pipeline, but they will anyways have to do that.

The only thing is that the dnf install -y glibc-langpack-en hack is a bit ugly. Maybe we should just put that on the container and update the container?