[qt] Move BitcoinGUI initializers to class, fix initializer order warning

Summary:
C++11-ize the code (move initializers to class, change 0 to nullptr where appropriate)

Backport of Core PR 13528

Test Plan:

make VERBOSE=1 check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1959