Windows file dialog: Clean up thread manually.

Do not use deleteLater() to delete the thread.

Task-number: QTBUG-36357

Change-Id: Ie7c87b92a7c73d5fbac01d4951d387ee2facd05c
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Fallback to QWidget::grab() if QScreen::grabWindow() fails.

QScreen::grabWindow() is not always reliable because it grabs from the
framebuffer. (The window might then be covered by other windows, e.g.
"Stays on top"-Windows, popups etc).
If QScreen::grabWindow() fails we therefore fallback to
QWidget::grab(). This will not grab from the frame buffer, but it will
ask the widget to render itself (with its current state) to a pixmap
and return it.
QWidget::grab() should usually return the expected pixmap, and the
pixmap it gives is not subject to the state of the window manager.

This means that both QScreen::grabWindow() *and* QWidget::grab()
must produce an unexpected pixmap in order for the test to fail.

Change-Id: I276554155bb1e5b510d2a2d43628d91669464fe2
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>