macosx: addons: add a new addon class for the model representation

Also loads the locally installed addons properly to mark these ones
as installed.

close #11476
vda: readd ref buffer flag (for ffmpeg variant only)
l10n: Taking po files from 2.1
l10n: Acoli update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Initial Amharic (Ethiopia) translation

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Aragonese update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Arabic update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Asturian update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Azerbaijani update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Belarusian update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Bulgarian update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Bengali (India) update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Bengali update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Breton update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Bosnian update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Catalan update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Initial Catalan (Valencian) translation

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Czech update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Welsh update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>
l10n: Danish update

Signed-off-by: Christoph Miebach <christoph.miebach@web.de>