feature: find "important" messages easier #1010

I am losing the overview a little in BM

e.g. currently I see only as ways for me:

• marking as unread (but this read-status can be lost again) or
• saving to a file (which brings me outside of BM)

e.g. I'd like to mark a message with a certain tag (so I could later find all those mails with tags) or move to a special folder in the messages UI