extensionPrefs: Rename to Extensions

... and make the application user-visible.

GNOME/gnome-shell#1968
6 jobs for !956 with appify-extension-prefs in 7 minutes and 2 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #577869
check_commit_log

00:00:23

passed #577871
eslint

00:00:56

passed #577870
js_check

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  Build
passed #577872
build

00:05:13

 
  Test
passed #577873
test

00:00:51

passed #577874
test-pot

00:00:39