Diaglib: Improve output of widgets.

- Make it possible to pass an optional root widget to dumpAllWidgets().
- Add option to output size constraints of widgets/windows.
- Output normal geometry of top levels.

Change-Id: Ib48809d070c5721fe4688a2ad39cb99f286618de
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>