Doc: update Editable Tree Model Example

- new screenshot with more up-to-date graphical look
- deleted itemviews-editabletreemodel.png from old folder
doc/src/images
- deleted unnecessary item in qtwidgets.qdocconf

Task-number: QTBUG-60635
Change-Id: I9b82fa027347fb0fd7adb86038abd32853e68c3a
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>