gui: Apply port mapping changes on dialog exit

Summary:
This commit does not change behavior. It is a prerequisite for NAT-PMP
support adding.

This is a backport of core#18077 [6/13]
https://github.com/bitcoin/bitcoin/pull/18077/commits/58e8364dcdc4e57b0caac09f8402e6535301de9b

Depends on D12073

Test Plan:
ninja all check-all

Run src/qt/bitcoin-qt, open the Options menu, go to Network tab, check"Map port using UPnP".
Restart src/qt/bitcoin-qt and check that the settings are persistent.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12074