The focus chain in widgets is implemented as a double-linked list,
using QWidgetPrivate::focus_next and focus_prev as pointers to the
next/previous widget in the focus chain.
These pointers are manipulated directly at many places, which is error
prone and difficult to read.
Build an abstraction layer and remove direct usage of focus_next and
focus_prev. Provide functions to insert and remove widgets to/from the
focus chain.
Add a test function to tst_QWidget.
Task-number: QTBUG-121478
Change-Id: Ide6379c0197137e420352a2976912f2de8a8b338
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
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