customize actions list in notification and message list bottom bar

[wwp]

There's already a 'show delete button' for notifications, and a 'visible message actions' but there's no real way to configure what actions/buttons are present in the notification banners and in the bottom actions panel (message view menu).
About the message list bottom bar, for instance (not in multi-selection mode), I don't care about the search or sort actions, but I do about the mark-all-read, mark-as-spam and delete actions, but K-9 mail doesn't allow customizing this. Same for the notification banners.

[cketti]

See #611 and #943