Merge #9890: Add a button to open the config file in a text editor

Summary:
9ab9e7d Add a button to open the config file in a text editor (Eric Shaw Jr)

Tree-SHA512: 1d13be9ac788a05a5116dbb3e1136ef65732dc2b5634547860612658109668922c9ea80b77bde4ba5beaa762d54f2a986a6064d4e34e963cdcd3d126a4eced37

Backport of Core PR9890
https://github.com/bitcoin/bitcoin/pull/9890/files

Test Plan:

make check
bitcoin-qt
# Settings -> Options
# Click button to open configuration file
# See popup info window -> click OK
# Config file should open in the default editor

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3219