[gui] intro: add prune preference

Summary:
Adds a checkbox to the introduction screen letting the user enable pruning from the start.
Disable checkbox when launched with -prune

Partial backport of Core PR16714 - part 4 of 5
Commit c8de347a9d6c88fe67d77aba6fcce1b7fd66791c
Depends on D7954

Test Plan:
Unmount the filesystem containing the bitcoin data, to trigger the intro screen to show on startup of bitcoin-qt.
Verify that you can modify the prune setting and that it is remembered on restart.
src/qt/bitcoin-qt -regtest

Repeat the procedure with the following command, verify that the checkbox is checked and disabled.
src/qt/bitcoin-qt -regtest -prune=2

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7955