[Development] Rationalizing qApp and qGuiApp

[Sze Howe Koh]

Currently,

* The qApp macro changes type depending on which headers are included,
and in what order. (If you #include <QGuiApplication> but instantiate
a QCoreApplication, qApp returns a QGuiApplication pointer)
* The documentation says that the qApp macro is only valid if a
QApplication was instantiated. [1]
* The qGuiApp macro doesn't change types like qApp; it is always a
QGuiApplication pointer.
* There is no equivalent of qGuiApp for QCoreApplication and QApplication.

To resolve this inconsistency, I propose that we do one of the
following for Qt 6:

A) Update the documentation to say that qApp can change types. Leave
the qApp macro as-is.

B) Leave the documentation as-is. Make the qApp macro disappear from
qcoreapplication.h and qguiapplication.h unless compatibility mode is
enabled.

C) Rename "QApplication" -> "QWidgetApplication", keeping the former
as a typedef. Leave the qApp macro as-is but deprecated. Add the
qCoreApp and qWidgetApp macros (analogous to qGuiApp)

Thoughts?


Regards,
Sze-Howe

[1] https://codereview.qt-project.org/c/qt/qtbase/+/174414