Qt6 port

[wwmayer]

Fix deprecation warnings with version 6.3 or 6.4

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[chennes]

This PR resolves those deprecation warnings for me. FYI, when compiling against 6.6.0a1 I get a new deprecation. Here is an example:

/home/chennes/FreeCAD/src/Mod/Sketcher/Gui/TaskSketcherElements.cpp:1083:13: warning: ‘constexpr typename std::add_const<_Tp>::type& qAsConst(T&) [with T = QToolButton*; typename std::add_const<_Tp>::type = QToolButton* const]’ is deprecated: Use std::as_const() instead. [-Wdeprecated-declarations]
 1083 |     qAsConst(ui->filterButton)->addAction(action);
      |     ~~~~~~~~^~~~~~~~~~~~~~~~~~
/media/chennes/Qt/qt6-install/include/QtCore/qttypetraits.h:50:45: note: declared here
   50 | constexpr typename std::add_const<T>::type &qAsConst(T &t) noexcept { return t; }
      |                                             ^~~~~~~~

[wwmayer]

TBH I don't understand the use of qAsConst in this context which was added with 15dd775.
IMO, it should be removed here.

[wwmayer]

@chennes
What about the line: qAsConst(ui->filterButton)->addAction(action); ? Shouldn't we replace it with ui->filterButton->addAction(action);

Btw, the same line occurs in TaskSketcherConstraints::TaskSketcherConstraints

[chennes]

Yes, I figured we'd do the forward-looking deprecations in a separate PR -- what we have now should work fine with Qt 6.4, I was testing on a version of 6.6 from git in my comment above.