Qt Core Changes in Qt 6

This epic captures the work planned in QtCore for Qt 6, with the individual user stories and subtasks documenting the details.

The overall goal for QtCore in Qt 6 is to

• general cleanup
  • get rid of legacy an deprecated code ()
  • Remove 3rd party components (QTBUG-80304)
  • review and remove static variables
• increase consistency and compatibility with the C++ standard library in containers and value types (QTBUG-77477)
  • std API adoption (QTBUG-77481)
  • build Qt with C++17 compiler mode (QTBUG-77479)
  • adopt smart pointer usage to indicate ownerships (QTBUG-80305)
  • consistently use View classes in Qt APIs
  • API overloads for ranges (QTBUG-80307)
• streamline the Qt object model and type system
  • Compile time resolution of meta type infos (QTBUG-75668)
  • Use new meta type if in meta objects (QTBUG-75670)
  • new property system (QTBUG-75672)
    • Revisit QObject, signals, slots and properties
    • Add support for QML style bindings from C++ (e.g: https://woboq.com/blog/property-bindings-in-cpp.html)
  • Improve meta object data (QTBUG-75674)
  • Investigate usage of clang by moc
• cleaner abstractions for I/O and similar operating system services
  • Investigate a QIODevice redesign (QTBUG-76492) (Descoped for 6.0)
  • (Investigate a "QPA for OS services") 
    • Not necessarily required for 6.0

Get rid of some third-party components:

• Get rid of legacy Text Codecs (EUC-JP, EUC-KR, GBK, JIS, Shift-JIS, TSCII, Big5, Big5-HKSCS) (QTBUG-75991)
• Get rid of copy of Public Suffix List / QUrl::topLevelDomain (QTBUG-80308)

Container improvements

• Unify QList and QVector (QList as an alias to QVector) ()
• QFlatMap (patch exists)
• using QPair = std:pair (deprecate, remove/replace) (QTBUG-80309)
• Find a fast hash implementation for QHash (std::unordered_map too slow) (QTBUG-80311)
• Use std::map to implement QMap (to be verified)

Add Qt5Compat to Qt 6 (QTBUG-80312)

• Get rid of QRegExp (implies unbootstrapping of qmake)
• Move QLinkedList out of QtCore

Move QStateMachine out of QtCore (QTBUG-80316)

Revisit Multithreading Technologies in Qt?

• Keep QThread and friends
• split QMutex into QMutex and QRecursiveMutex (done)
• Deprecate QtConcurrent?
• Fix QFuture API or deprecate and move to QtConcurrent
  -> decision process (QTBUG-79283)
• std::future is not considered to be complete (including ++20) and hence not an option -> no Qt 6 relevance

Make QDir and friends consistent about their handling of absolute/relative for MS-paths; this may involve some compatibility breakage. The core of the problem is that QFileSystemEntry isn't consistent with itself.
--> Discussion 18.11.2019: Move into a separate task and assign to Eddy

Other changes:

• MS absolute vs relative file path behavior change
• QByteArray changes (QTBUG-64747, )
• naked const char * pointer to be removed (except from QByteArray and string handling classes)
• Investigate usage of qsizetype by Qt containers