KWin::rootWindow() uses the property to resolve the value instead of
using QX11Info. In practice this doesn't change anything at the moment,
but allows kwin_wayland to provide a root window without depending on
QX11Info.
It's a convenient class to encapsulate the ICCCM WM_SIZE_HINT.
Instead of exposing just the properties it provides accessors for
the interesting parts and applies sanity checks.