Merge #12489: Bugfix: respect user defined configuration file (-conf) in QT settings

Summary:
a6e6e39a8b Bugfix: respect user defined configuration file (-conf) when open conf. file from QT settings (Jonas Schnelli)

Pull request description:

Fixes #12488.

In master, opening the configuration file from the GUI settings will always open the file "bitcoin.conf" regardless of the `-conf=` settings.
This PR makes the GUI settings open configuration file function respect the `-conf` option.

Tree-SHA512: fb54cc699b4d2a3947f749fdf5f1a51251ffd67d0f6c6a937a5b80f0ba5a5c1085d0eef190453bbc04696d4d76c2c266de0fe9712e65e4bb36116158b54263d4

Backport of Core PR12489
https://github.com/bitcoin/bitcoin/pull/12489/files

Test Plan:

make check

echo "testnet=1" >> ~/.bitcoin/bitcoin2.conf
bitcoin-qt -conf=bitcoin2.conf
# Verify that Options -> Open Configuration File opens bitcoin2.conf (opens bitcoin.conf on master)

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3846