Fix selling improvements for multiple cities at once

[blabber]

Closes #2483

[blabber]

I will keep this as a draft for now. I aim to factor out the selling of improvements into it's own function/method before merging it.

[lmoureaux]

I think it would make sense to unify this with eco_report::sell_buildings in view_economics.{h,cpp}.

[blabber]

I think it would make sense to unify this with eco_report::sell_buildings in view_economics.{h,cpp}.

Unifying the behavior? Showing a sell results dialog after selling the improvements from the cities view seems like a good idea, and I can do that. Unifying the code on the other hand...

[lmoureaux]

Unifying code is probably better. The function sell_all_improvements in repodlgs_common.cpp could take a city list parameter (and be renamed accordingly), and maybe boilerplate for the "sell results" dialog could be shared as well. Not sure about the confirmation dialog since the message is slightly different: sell every Temple vs sell the Temple.

If you don't see how to do it we can open a refactoring issue.

[blabber]

I followed your advice and unified the code used by both views to sell city improvements.

We could also generate the question and result dialog in this central function, but I am not sure if this is considered good practice, so I left it out for now. Let me know what you think.

[lmoureaux]

Thanks! I think we can remove the draft flag (and then I'll want to check the functionality one last time).

[lmoureaux]

Small nitpick on the working, else ok!