Stabilize tst_scrollbar.

- Use a separate widget for each test.
- Replace hard-coded timeouts by qWaitForWindowExposed.
- Center windows on screen.

Change-Id: I45ae306fd23348ee5dec3849e64ec98843e8bf32
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Windows: Use focusWindow() as native dialog parent when 0 is passed.

A parent is required, otherwise the application gets deactivated when
the dialog closes. The same is done in Qt 4.

Task-number: QTBUG-36039

Change-Id: I56688b3259f1d207edd1bec499afd1365ae511dc
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>