appDisplay: Look up directory- instead of category translations

Translations are provided by .directory files, so trying to look
up a category name without the suffix will always fail.

Commit 343b3351 tried to fix this previously by changing the
saved keys, but that broke existing translatable folders.

Appending the .directory suffix for the lookup instead fixes the
issue without regressing non-custom folders.

GNOME/gnome-shell#2623
9 jobs for !1196 with fix-fixed-folder-name in 6 minutes and 19 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #669861
check_commit_log

00:00:22

passed #669863
eslint

00:01:11

passed #669862
js_check

00:00:45

passed #669865
no_template_check

00:00:31

passed #669864
potfile_check

00:00:44

 
  Build
passed #669866
build

00:03:34

passed #669867
flatpak
flatpak

00:01:10

 
  Test
passed #669868
test

00:00:52

passed #669869
test-pot

00:01:31