settings/addables buttons have access key conflict and partial i18n behavior

[Kit Blake]

The settings button in the Properties screen has an access key conflict with the
save button for the metadata (alt-s). Let's make the access keys for it and the
addables button the same as in the subscreens (6 & 7). This also means removing
the underlining from the buttons in the Properties screen.

Seems like the i18n of these buttons is not working properly. In the Properties
screen they appear in english, and in the subscreens only the unselected button
gets translated.

While you're at it, please add German translations, as they're missing.
Although, if 'addables' can only be translated as Erweiterungseinstellungen,
maybe we'd better leave the button as is :-)

[Andy Altepeter (aaltepet)]

At some point, the access keys were changed to alt-e and alt-d, which don't conflict with other keys on the screen. Also, the translations appear to be working now.