Having windows ID as guint32 is a mistake. Different systems have
different protocols. In Wayland in particular, Windows handles are
exchanged as strings. What this commit does is the following:

In core:

- get_window_id() virtual function in core GimpProgress is changed to
  return a GBytes, as a generic "data" to represent a window differently
  on different systems.
- All implementations of get_window_id() in various classes implementing
  this interface are updated accordingly:
  * GimpSubProgress
  * GimpDisplay returns the handle of its shell.
  * GimpDisplayShell now creates its window handle at construction with
    libgimpwidget's gimp_widget_set_native_handle() and simply return
    this handle every time it's requested.
  * GimpFileDialog also creates its window handle at construction with
    gimp_widget_set_native_handle().
- gimp_window_set_transient_for() in core is changed to take a
  GimpProgress as argumen...