xwayland: Add mapped argument to xwayland_view_create()

xwayland: Add mapped argument to xwayland_view_create()

This is a trivial cleanup to make xwayland_view_create() symmetrical with
xwayland_unmanaged_create(), and avoid the need to access view->impl from
xwayland-unmanaged.c.

The return value of xwayland_view_create() is no longer user, so return void.

No functional change.