Manually check for xgettext as we need it for NLS support.

See https://github.com/mesonbuild/meson/issues/8436 for the upstream bug this is working around.

Also fixed an issue where the appdata file was causing configuration to fail if nls was turned off.

Testing Done:
Ran locally with `-Dnls=true` and `-Dnls=false`.

Reviewed at https://reviews.imfreedom.org/r/548/