fdo#80373: change wording to 'Area Transparency Mode'

... in order not to be imprecise. Also, restore Title Case: enough trouble
(for l10n teams especially) is this case thing, let's be consistent.
See https://bugs.freedesktop.org/80373#c12

Change-Id: Icdd61c42ed294c41d9ec5f671168a606d0f93532
Reviewed-on: https://gerrit.libreoffice.org/10314
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>