Save/restore main selection from/to strings

Always consider that the first selection in the list is the main
one, save selections that way.

This approach was suggested by PR #1786 but the implementation here
is different, and is used more generally whenever we save selections
to strings.

This is also the prefered way to work only on the main selection:
save selections with Z, reduce to main with <space>, restore with z.

Closes #1786
Fixes #1750

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