So far the tracking for cursor shape was done incorrectly on Wayland by
only listening to X11 cursor changes. That's from a time when
KWin/Wayland was still run on top of an X server.
Nowadays the Platform tracks cursor shape changes and emits changes to
it. Xwayland cursor changes go through the normal Wayland way so it's
just one way to get it on Wayland.
This change adds the required connect and changes the signal signature
in Cursor by dropping the serial argument. No user of the signal uses
the argument and on Wayland we don't have a serial for the cursor. So it
can be dropped.