wayland: Fix cross cursor in Xwayland apps

Startup code in plasmashell was changed so xsetroot is not called
anymore, which is sort of fine.

Unfortunately (or not?), it exposed a bug in kwin. Cursor::x11Cursor()
only works in the standalone X11 session.

On Wayland, Cursor::x11Cursor() will return XCB_NONE which results in
seeing cross cursor when there should be arrow cursor.

This change moves xcb_cursor_t look up code from X11Cursor to the base
Cursor class. In hindsight, I would like to introduce a window manager
class where the xcb cursor and other x11 specific code can be moved in
the future for better encapsulation of platform-specific code.

CCBUG: 442539


(cherry picked from commit ca1c72dd)