COMP: Fix QModelIndex::child() deprecation warnings

[jcfr]

This commit introduces ctk::modelChildIndex() function so that the
same code compiles without deprecation warnings pre and post Qt 5.8.

It fixes warnings like the following:

  /path/to/S-r/CTK/Libs/DICOM/Core/ctkDICOMModel.cpp:745:32: warning: 'child' is deprecated: Use QAbstractItemModel::index [-Wdeprecated-declarations]
        this->setChildData(index.child(i,0), value, role);
                                 ^
  /path/to/Support/Qt/5.15.2/clang_64/lib/QtCore.framework/Headers/qabstractitemmodel.h:71:5: note: 'child' has been explicitly marked deprecated here
      QT_DEPRECATED_X("Use QAbstractItemModel::index") inline QModelIndex child(int row, int column) const;
      ^
  /path/to/Support/Qt/5.15.2/clang_64/lib/QtCore.framework/Headers/qglobal.h:294:33: note: expanded from macro 'QT_DEPRECATED_X'
  #  define QT_DEPRECATED_X(text) Q_DECL_DEPRECATED_X(text)
                                  ^
  /path/to/Support/Qt/5.15.2/clang_64/lib/QtCore.framework/Headers/qcompilerdetection.h:675:55: note: expanded from macro 'Q_DECL_DEPRECATED_X'
  #    define Q_DECL_DEPRECATED_X(text) __attribute__ ((__deprecated__(text)))
                                                        ^

References:

• https://doc.qt.io/qt-5/qmodelindex-obsolete.html#child

• Commit deprecating the function: qt/qtbase@8dc45d5#diff-42185f7432a3738dc6ee6f3b1a0cc5db9d698da9f424f55b43c2070007b7fef7

• .. and the corresponding trivial (1 line) implementation is here for 4,8 or here for 5.15

[lassoan]

Looks perfect, thank you.