Fix deprecation warnings in Qt Tools and compile with QT_DISABLE_DEPRECATED_BEFORE=0x060000

Patches for QTBUG-104857 introduce a lot of forgotten deprecation warnings for Qt 6.0. After all these patches are applied locally, qttools throws a bunch of deprecation warnings from the designer component.

The scope of this task is to fix those warnings, and then try to build Qt Tools without the deprecated APIs. This can be done by specifying QT_DISABLE_DEPRECATED_BEFORE=0x060000

Most, if not all, of the deprecation warnings come from QAction's menu-related APIs.

Most of QAction's APIs have replacements. Exception is QAction::setMenu(), but it can be fixed by using the private API:

  QAction *myAction = ...;
  QMenu *myMenu = ...;
- myAction->setMenu(myMenu);
+ QActionPrivate *p = QActionPrivate::get(myAction);
+ p->setMenu(myMenu);

That shouldn't be an issue because we are anyway linking with Qt::GuiPrivate in all such cases, AFAICT.