Move the get_preferred_width/height() and allocate() vfunc
implementations of WindowPreview to C, subclassing the C GObject from
JS.
This gets us another significant performance gain, allocating a
workspace with 20 windows now only takes 1.2 ms.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1743>
