Intro: Never change the prune checkbox after the user has touched it

[luke-jr]

Re-PR from bitcoin/bitcoin#18729

Now includes a bugfix too (-prune=2+ disabled the checkbox, but -prune=0/1 did not; this behaviour is necessary since -prune overrides GUI settings)

[hernanmarino]

cr ACK 76cb089 and tested ACK

[pablomartin4btc]

Title and description mismatch of the PR confuse me, I tested bitcoin-qt before and after the change and I don't see the difference in the behaviour (from the UI point of view, haven't checked the proper prunning).
I've ticked & unticked the prune checkbox and set a value, verifying the value is persisted in the config. Also combined the previous starting qt with -prune param set to 0, 1 and 600.
Perhaps I've some config that bypass this change? Could you please clarify? Sorry if I miss anything too obvius here.

[hebasto]

-prune=2+ disabled the checkbox

Looks like a bug in the master branch, as it should be like that:

$ ./src/bitcoind -prune=2
Error: Prune configured below the minimum of 550 MiB.  Please use a higher number.

[hebasto]

@ryanofsky Could you be interesting in reviewing of this PR?

[hebasto]

Closing due to a long period of inactivity here. Feel free to reopen.

[achow101]

Reopening by request

[luke-jr]

Rebased

[hebasto]

ACK bee0ffb, both commits are improvements of the current behaviour. Tested on Ubuntu 23.10.