[gui] add explicit prune setter

Summary:
This makes it possible to enable pruning after the OptionsModel has been initialized and reset.

Partial backport of Core PR16714 - part 2 of 5
Commit 1957103786f97135f35ababc97efa1b481865eb0
Depends on D7952

Test Plan:
ninja && ninja check
Run src/qt/bitcoin-qt -regtest, change prune settings and check that settings are remembered after restart.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7953