Add mechanism in order to “host” a control with a Tab-focus regime different from QQuickItem

In a read-only document (e.g. web browser), it is common to be able to Tab through links. Or maybe you would like to tab through all objects in a Qt Quick 3D scene. Today, we don’t have any mechanism to support that, since <Tab> will just move focus to the next focusable QQuickItem.

Could be useful for:

• Custom tab focus handling in a Textbrowser control (e.g. Tab through links)
• Custom tab focus handling in a TableView/GridView/ListView (e.g. move to next/previous cell)
• Hosting a QWidget form inside a QQuickItem (or any other framework with tab focus)
• A quick view hosted in a window container, to return focus to widgets, when pressing a tab key wraps the quick focus chain.

Suggestion

A mechanism similar to

virtual QWidget::focusNextPrevChild() 

I don’t think it is important to expose this to QML, but if we want we could add a JS callback later (that is called from the virtual if the callback exists).