On Mac only editable text input should receive tab focus

It's for all items with "readOnly: false" and has "text" property.

[ChangeLog][QtQuick] Mac: any editable text input will get tab focus
when "Text boxes and lists only" option was selected.

About "Text boxes and lists only", see commit
06332df7438c8d2215b02f1e01ce2ed28a49a320.

Task-number: QTBUG-38004
Change-Id: I73947b71b2fec69a66e122514d440656f4650e99
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>