Create imported_names_layout option

[behnam]

This is a WIP for #788. I have implemented the option using bare ListTactic, which now looking at the outcome, I see that it needs more work. But there are some open questions now:

• Do we want to support anything like "Visual" mode? I think not, but we need to make sure, to some level.

• Would a trailing comma be optional in this list? Do we need to add an option for it? (Again, I think No and No.)

• And the important question: should I just hard-code the "Block" style for "Vertical" option here, or should we split this option into two, or maybe create a new type for it, so we can have "Horizontal" and "Block" in one option. (I don't have an answer for this myself, but until I hear back, will try to get it work as "Block" with the simplest diff possible.)

Fixes #788

[nrc]

Do we want to support anything like "Visual" mode? I think not, but we need to make sure, to some level.

I don't think this is necessary

Would a trailing comma be optional in this list? Do we need to add an option for it? (Again, I think No and No.)

Ideally we should have an option (on by default), but I don't think it is necessary for this PR.

And the important question: should I just hard-code the "Block" style for "Vertical" option here, or should we split this option into two, or maybe create a new type for it, so we can have "Horizontal" and "Block" in one option. (I don't have an answer for this myself, but until I hear back, will try to get it work as "Block" with the simplest diff possible.)

I think hardcoding Block for Vertical is a good idea. If someone wants visual/vertical, we can add it in later. Starting simple SGTM.

[nrc]

@behnam how's it going with this? Did you have any more questions?

[behnam]

No questions, @nrc. I spend a little time on it after your feedback, but the code was getting out of hand with all the special handling. Not sure when I can get back to it again. Please feel free to take over if it can be done easily. (I haven't found the simple solution yet.)

[topecongiro]

Closing this in favour of #1785. @behnam Thanks for the PR!