Use window() instead of obsolete topLevelWidget()
Use window() instead of obsolete topLevelWidget()

Summary:
`topLevelWidget()` has been deprecated since Qt4.8, and returns `window()`

Backport of Core [[https://github.com/bitcoin/bitcoin/pull/14801 | PR14801]]

Test Plan: `ninja ckeck-bitcoin-qt`

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7679

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