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 [[https://github.com/bitcoin/bitcoin/pull/16714 | PR16714]] - part 4 of 5
Commit [[https://github.com/bitcoin/bitcoin/pull/c8de347a9d6c88fe67d77aba6fcce1b7fd66791c | c8de347a9d6c88fe67d77aba6fcce1b7fd66791c]]
Depends on D7954