po: restore the ability to translate strings in .settings files
Commit e9fa470e ("po/: Modernize i18n.") inadvertently remove the ability to translate strings in .settings files, which are used to describe each plugin's name and search keywords, amongst others. A few plugins refer to the plugin's name via this file, to avoid repeating themselves, which make the missing string visible.
Re-introduce the tooling used to extract those strings into the .pot file, while fit them into the new, more modern CMakeLists.txt file. A small change is also made to make it easier to understand where the strings are extracted from, to reduce the chance that they're removed again.