automatically download adjacent ways when splitting a line

[k-yle]

Closes #10469, Closes #10316, Closes #11601

This error message has been removed:

and replaced with a popup that allows you to download the adjacent relation members and then continue the split operation:

[k-yle]

I think a nicer solution would be to make the popup appear instead of this error, so that splitting a line is a more streamlined process.

But I thought I'd open a PR first to get feedback

[tyrasd]

in the future this functionality could potentially made more generic and reused for things like #9020

[k-yle]

all the conflicts from 4dc4e7d and e25af7c should be fixed now