invert the display_next flag if user holds the shift modifier key while deleting a message

[the0ne]

When deleting a message, usually I don't want to go to the next message, but sometimes I want to.
Other users might want to usually go to the next message, but sometimes they possibly don't want to.

With this small addition, this can be easily achieved:

1. Set your preference of "display_next" as usual.
2. Whenever you want the opposite behaviour when deleting a mail, just hold the 'Shift' modifier key while doing so.

[alecpl]

It does not make sense to me. Shift key already has a meaning on delete.

[the0ne]

Shift key already has a meaning on delete.

Ah, yes - I see. The shift key is used for a specific case where it's about deleting without moving to trash. I didn't notice this as it's clearly not my use case, I always have 'flag_for_deletion' active.

It does not make sense to me.

You probably don't use the mail list in the way I do. By keeping mail unread it's marked as 'open' with the meaning of 'not yet done'. Often I need to delete newer (server log) emails but keep the newest message unread. So either I'd have to turn display_next on, then I'd have to remember to every time manually set the newest message to unread again. Or I'd have to turn display_next off, then I'd have to manually go to the next server log email after deleting one of them. Both is not convenient. In horde, the behavior was exactly like it's with the patch now, which I like a lot as a very convenient solution.

Maybe I could change this so that the new code is only active if 'flag_for_deletion' is active - would you like that better?

[alecpl]

If 'flag_for_deletion' is active 'display_next' does not apply as the message is not being removed from the list. So, it does not make much sense. In this mode you can just press Del key and then Arrow-Down if you wish or not.

That being said, maybe use Ctrl key instead of Shift and we're good. I don't know. This may come in handy with #9357 if we decide it's a good idea.