[Development] Old platform-specific functions

[Sze Howe Koh]

Hi all,

There's a list of platform-specific functions from Qt 4:
http://doc.qt.io/qt-5/exportedfunctions.html

It looks like most of these functions are now gone. The non-existing
functions should be removed from the documentation, but 3 functions
remain in the code base:

* qt_mac_secure_keyboard() is marked Q_DEAD_CODE_FROM_QT4_MAC (qlineedit.cpp)
* qt_mac_set_dock_menu() is marked QT_DEPRECATED, and is slated for
removal in Qt 6 (qmenu.h)
* qt_set_sequence_auto_mnemonic() is not marked as dead/deprecated in any way.

Should any of these be left in the documentation? (Note that
qt_set_sequence_auto_mnemonic() is currently documented as a way to
get non-standard behaviour on OS X:
http://doc.qt.io/qt-5/qshortcut.html#details )


Regards,
Sze-Howe