let adding transport fail in case "shared account options" are set

[r10s]

if one of the following "shared account options" are enabled, adding a transport should fail.

error returned by the json call should be sth like:

Sharing accounts with classic email clients is unsupported.
To add a transport, please check the following advanced settings:

- Disable "Move to Delta Chat Folder"
- Disable "Only Fetch From Delta Chat"
- Set "Show Classic E-Mail" to "All"

we want to fade out these options anyways and do not want to pull in their complexity to multi transport.

the other way round, when multiple transports are defined, these options should not be changable, at best not appear in UI - this is probably easily possible if is_chatmail is set (UI do hide these optionss then)

[link2xt]

This is already part of #7348, mvbox_move and only_fetch_mvbox already prevent you from adding a second transport.

Need to also do this for "show classic e-mail".

[link2xt]

Closed with #7348