QMainWindow::tabifiedDockWidgets() - Not accurate

The method QMainWindow::tabifiedDockWidgets() is not accurate. It may return docks from a previous tabification.

Steps to Reproduce:
1. Download attached dockwidgets.zip, extract, and build
2. Launch the sample application
3. Press the Refresh Tab Counts button
4. Notice that all dock widgets list 0 tab "buddies" (correct)
5. Pull off dock #2 (bottom dock) and tabify with dock #3.
6. Press the Refresh Tab Counts button
7. Notice that dock #2 and dock #3 list 1 tab buddy (correct).
8. Pull off dock #2, but do NOT dock again. Leave dock #2 floating.
9. Press the Refresh Tab Counts button
10. Notice that dock #2 shows 0 tab buddies, but dock #3 still lists 1 tab buddy (incorrect).