wayland: Fix blank window thumbnails

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.