COMP: Fix qt5_get_moc_flags/qt5_create_moc_command deprecation CMake warnings
COMP: Fix qt5_get_moc_flags/qt5_create_moc_command deprecation CMake warnings

This commit updates ctkMacroGenerateMocs to internally use
qt5_generate_moc and fix deprecation warnings reported when
building against Qt >= 5.15.0

See https://doc.qt.io/qt-5/qtcore-cmake-qt5-generate-moc.html
and https://doc.qt.io/qt-5/cmake-command-reference.html#qt5-core

Example of warnings:

  CMake Warning (dev) at /path/to/Support/Qt/5.15.2/clang_64/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:44 (message):
    qt5_get_moc_flags is not part of the official API, and might be removed in
    Qt 6.
  Call Stack (most recent call first):
    /path/to/Support/Qt/5.15.2/clang_64/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:86 (_qt5_warn_deprecated)
    /path/to/Projects/S-r/CTK/CMake/ctkMacroGenerateMocs.cmake:8 (QT5_GET_MOC_FLAGS)
    /path/to/Projects/S-r/CTK/CMake/ctkMacroGenerateMocs.cmake:43 (QT4_GENERATE_MOCS)
    Modules/Loadable/VolumeRendering/Testing/Cxx/CMakeLists.txt:41 (QT5_GENERATE_MOCS)

  CMake Warning (dev) at /path/to/Support/Qt/5.15.2/clang_64/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:44 (message):
    qt5_create_moc_command is not part of the official API, and might be
    removed in Qt 6.
  Call Stack (most recent call first):
    /path/to/Support/Qt/5.15.2/clang_64/lib/cmake/Qt5Core/Qt5CoreMacros.cmake:120 (_qt5_warn_deprecated)
    /path/to/Projects/S-r/CTK/CMake/ctkMacroGenerateMocs.cmake:32 (QT5_CREATE_MOC_COMMAND)
    /path/to/Projects/S-r/CTK/CMake/ctkMacroGenerateMocs.cmake:43 (QT4_GENERATE_MOCS)
    Modules/Loadable/VolumeRendering/Testing/Cxx/CMakeLists.txt:41 (QT5_GENERATE_MOCS)
  This warning is for project developers.  Use -Wno-dev to suppress it.

Co-authored-by: Andras Lasso <bf5e972943f0e90adb0d9bfa939dcc72ce410bec@queensu.ca>

commontk/CTK,commontk/CTK,commontk/CTK,commontk/CTK