Merge #12928: qt: Initialize non-static class members that were previously neither initialized where defined nor in constructor

Summary:
3fdc5fe Make sure initialization occurs in the constructor (practicalswift)
1e7813e Remove redundant initializations from the constructor (practicalswift)
f131872 Initialize non-static class members where they are defined (practicalswift)
73bc1b7 Initialize editStatus and autoCompleter. Previously not initialized where defined or in constructor. (practicalswift)

Pull request description:

Initialize variables previously neither defined where defined nor in constructor:
* `editStatus`
* `autoCompleter`

Also; initialize non-static class members where they are defined in accordance with developer notes.

Tree-SHA512: 84f0cb87ec8394ed7641bfa0731be2ec72e6a920e00ae206ff89e2e7c960358f603c52878311b24601a33aa7cba6ea4f9a78a8ade88112dea0f41efb08e84e25

Backport of Core PR12928
https://github.com/bitcoin/bitcoin/pull/12928/

Test Plan:

make check
test_runner.py
./bitcoin-qt

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3937