[Accessibility] Add a flag to remove a QWidget from the accessibility tree

There are cases in which QWidgets are non focusable and added just for cosmetic purposes (icons, indicators etc) and there is no way to mark the widget so that it won't appear in the accessibility tree. This is possible with the aria-hidden attribute.

It would be useful to have a flag on QWidget which could be used in childWidgets in qaccessiblewidget.cpp to avoid adding the child.

Similarly it would be very handy to have an option to "inline" the children of a widget in the accessibility tree, since there are many cases of widget wrappers, or widgets used just to group children in a layout, that shouldn't be exposed in the accessibility tree as group.