gui: Drop boost::scoped_array and use wchar_t API explicitly on Windows

Summary:

• Drop boost::scoped_array
• gui: get special folder in unicode

This is a backport of Core PR13734

Test Plan:

make check

Run bitcoin-qt on win64

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5889