xwayland: Better document/assert view/surface association

xwayland: Better document/assert view/surface association

Each XWayland view is paired with a particular wlr_xwayland_surface and
its lifetime is tied to that surface.  This condition in handle_map():

    if (xsurface != view->xwayland_surface)

could never be true since the view is only registered to receive the
"map" signal from view->xwayland_surface, and no other.  So the code
updating view->xwayland_surface in handle_map() was dead.

So let's clean things up a little:

- Remove the dead code
- Add some comments, and slightly rearrange code to match
- Add/update assert()s in signal handlers for consistency
- Pass xsurface as <data> when calling handle_unmap() and
  handle_destroy() explicitly, to be consistent