[backport] qt: Replace functions deprecated in Qt 5.13 #16701

Summary:
c6dd32da697e5a8052cbabe8c7605d27c43a8dfb qt: Replace obsolete functions of QDesktopWidget (Hennadii Stepanov)
1260ecd812e35185898fd555ad3e01d019072bcf qt: Add TextWidth() wrapper (Hennadii Stepanov)

Pull request description:

The following functions are obsolete in Qt 5.13:

• [QFontMetrics::width()](https://doc.qt.io/qt-5/qfontmetrics-obsolete.html#width)
• [QDesktopWidget::availableGeometry()](https://doc.qt.io/qt-5/qdesktopwidget-obsolete.html#availableGeometry)
• [QDesktopWidget::screenGeometry()](https://doc.qt.io/qt-5/qdesktopwidget-obsolete.html#screenGeometry)

This PR replaces them and does not change behavior.

This is a backport from Core PR16701

Test Plan:

ninja check
run bitcoin-qt and click around to see everything is sane

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5369