Do not set Qt::WindowContextHelpButtonHint anymore by default

We should hide the Context Help Button by default in dialogs.

Using the context help / "What's this" mode is an outdated paradigm that is rarely used these days. Still, QDialogs on Windows and KDE/LInux by default still show the ? button that triggers the mode. Just that most dialogs do not provide the required metadata to show anything sensible then.

In Qt 5.8 we added an opt-in global solution to disable the context help button by default: Qt::AA_DisableWindowContextHelpButton. In Qt 6, this should become the default and Qt shouldn't set }}{{Qt::WindowContextHelpButtonHint{{ for all dialogs by default.}}