With the introduction of stripped down window items, the WindowPixmap
objects no longer form a hierarchy. WindowPixmap::children() method was
removed.

Surprisingly, the removal of the children() method didn't result in a
compilation error because the QObject class has a method with the same
name.

Currently, a window pixmap will have no QObject children even if the
associated wayland surface has child sub-surfaces. This may result in
blank thumbnails of apps that use sub-surfaces, e.g. Firefox. In order
to fix that issue, we need to check if there are child items instead.