The X11 specific tests need to run on QT_QPA_PLATFORM xcb otherwise
they will crash. To enforce this without having to replace QTEST_MAIN
a Q_CONSTRUCTOR_FUNCTION is used to invoke a function which does nothing
except setting the env variable.
KWin::connection() 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 an xcb connection without depending
on QX11Info.
As we cannot make xcb_connection_t* available as a metatype, the
property's type is set to void*.
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.
