Merge #16701: qt: Replace functions deprecated in Qt 5.13

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.

  Here are some excerpts from the master build log:
  ```
  qt/bitcoingui.cpp: In constructor ‘BitcoinGUI::BitcoinGUI(interfaces::Node&, const PlatformStyle*, const NetworkStyle*, QWidget*)’:
  qt/bitcoingui.cpp:84:57: warning: ‘const QRect QDesktopWidget::availableGeometry(int) const’ is deprecated: Use QGuiApplication::screens() [-Wdeprecated-declarations]
           move(QApplication::desktop()->availableGeometry().center() - frameGeometry().center());
                                                           ^
  In file included from /home/hebasto/Qt/5.13.0/gcc_64/include/QtWidgets/QDesktopWidget:1:0,
                   from qt/bitcoingui.cpp:43:
  /home/hebasto/Qt/5.13.0/gcc_64/include/QtWidgets/qdesktopwidget.h:88:67: note: declared here
       QT_DEPRECATED_X("Use QGuiApplication::screens()") const QRect availableGeometry(int screen = -1) const;
                                                                     ^~~~~~~~~~~~~~~~~
  ```
  ```
  qt/bitcoingui.cpp:1410:74: warning: ‘int QFontMetrics::width(const QString&, int) const’ is deprecated: Use QFontMetrics::horizontalAdvance [-Wdeprecated-declarations]
           max_width = qMax(max_width, fm.width(BitcoinUnits::longName(unit)));
                                                                            ^
  In file included from /home/hebasto/Qt/5.13.0/gcc_64/include/QtWidgets/qwidget.h:50:0,
                   from /home/hebasto/Qt/5.13.0/gcc_64/include/QtWidgets/qdialog.h:44,
                   from /home/hebasto/Qt/5.13.0/gcc_64/include/QtWidgets/QDialog:1,
                   from ./qt/optionsdialog.h:8,
                   from ./qt/bitcoingui.h:12,
                   from qt/bitcoingui.cpp:5:
  /home/hebasto/Qt/5.13.0/gcc_64/include/QtGui/qfontmetrics.h:108:9: note: declared here
       int width(const QString &, int len = -1) const;
           ^~~~~
  ```
  ```
  qt/splashscreen.cpp: In constructor ‘SplashScreen::SplashScreen(interfaces::Node&, Qt::WindowFlags, const NetworkStyle*)’:
  qt/splashscreen.cpp:127:50: warning: ‘const QRect QDesktopWidget::screenGeometry(int) const’ is deprecated: Use QGuiApplication::screens() [-Wdeprecated-declarations]
       move(QApplication::desktop()->screenGeometry().center() - r.center());
                                                    ^
  In file included from /home/hebasto/Qt/5.13.0/gcc_64/include/QtWidgets/QDesktopWidget:1:0,
                   from qt/splashscreen.cpp:24:
  /home/hebasto/Qt/5.13.0/gcc_64/include/QtWidgets/qdesktopwidget.h:79:67: note: declared here
       QT_DEPRECATED_X("Use QGuiApplication::screens()") const QRect screenGeometry(int screen = -1) const;
                                                                      ^~~~~~~~~~~~~~
  ```

ACKs for top commit:
  jonasschnelli:
    utACK c6dd32da697e5a8052cbabe8c7605d27c43a8dfb

Tree-SHA512: deb7bcbf86e1dcc6508bd91288772c2fe8811db79fa2011de37d0469cdd094fbf7fd8c4512c607bed0bd08dc2968e893c0bbc190732c43c69ed1085259df766c

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