ci: Include .ui files in POTFILES check

As the previous commit shows, just checking source files isn't
enough. Extending the existing check to .ui files should make
sure that we catch that kind of breakage in the future before
it hits translators.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2173>

.gitlab-ci/check-potfiles.sh

@@ -1,10 +1,14 @@
 #!/usr/bin/env bash
 
 srcdirs="src subprojects/extensions-tool"
+uidirs="js subprojects/extensions-app"
 
 # find source files that contain gettext keywords
 files=$(grep -lR --include='*.c' '\(gettext\|[^I_)]_\)(' $srcdirs)
 
+# find ui files that contain translatable string
+files="$files "$(grep -lRi --include='*.ui' 'translatable="[ty1]' $uidirs)
+
 # filter out excluded files
 if [ -f po/POTFILES.skip ]; then
   files=$(for f in $files; do ! grep -q ^$f po/POTFILES.skip && echo $f; done)