So far the effects could just use the connection() and rootWindow()
provided by kwinglobals. Thus an internal detail from KWin core is
accessed directly.

To be more consistent with the rest of the API it's wrapped through the
EffectsHandler and with a convenient method in Effect.

The connection() is provided as xcbConnection() to free the very generic
name connection which could create confusion once we provide a wayland
connection to the Effects.

The rootWindow() is provided as x11RootWindow() to indicate that it is
for the X11 world.

REVIEW: 117597