[Qt] Fix deprecated QString::SplitBehavior (now Qt::SplitBehavior)

Summary:
This diff adds a GUIUtil function to split a QString into a list,
skipping the empty parts. It factorizes the check for Qt version and
removes the associated deprecation warning.

Test Plan:
With Qt 5.5.1 and 5.15:

ninja all check

Check the warning is gone.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6605