build: Use i18n.merge_file() for plugin files

Even though we still need to preserve the *.plugin.desktop.in workaround, we can easily use Meson's native i18.merge_file() to generate translations, instead of the custom_target(). This isn't groundbreaking, but it's a nice little cleanup.

Switch plugins to generate translations using i18.merge_file(). This can be tested with "$ ninja totem-update-po" in the build directory, the translation files still contain strings from the *.plugin.desktop.in files.