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 argument (instead of a guint32 ID), requests and
  process the ID itself, according to the running platform. In
  particular, the following were improved:
  * Unlike old code, it will work even if the window is not visible yet.
    In such a case, the function simply adds a signal handler to set
    transient at mapping. It makes it easier to use it at construction
    in a reliable way.
  * It now works for Wayland too, additionally to X11.
- GimpPdbProgress now exchanges a GBytes too with the command
  GIMP_PROGRESS_COMMAND_GET_WINDOW.
- display_get_window_id() in gimp-gui.h also returns a GBytes now.

PDB/libgimp:

- gimp_display_get_window_handle() and gimp_progress_get_window_handle()
  now return a GBytes to represent a window handle in an opaque way
  (depending on the running platform).

In libgimp:

- GimpProgress's get_window() virtual function changed to return a
  GBytes and renamed get_window_handle().
- In particular GimpProgressBar is the only implementation of
  get_window_handle(). It creates its handle at object construction with
  libgimpwidget's gimp_widget_set_native_handle() and the virtual
  method's implementation simply returns the GBytes.

In libgimpUi:

- gimp_ui_get_display_window() and gimp_ui_get_progress_window() were
  removed. We should not assume anymore that it is possible to create a
  GdkWindow to be used. For instance this is not possible with Wayland
  which has its own way to set a window transient with a string handle.
- gimp_window_set_transient_for_display() and
  gimp_window_set_transient() now use an internal implementation similar
  to core gimp_window_set_transient_for(), with the same improvements
  (works even at construction when the window is not visible yet + works
  for Wayland too).

In libgimpwidgets:

- New gimp_widget_set_native_handle() is a helper function used both in
  core and libgimp* libraries for widgets which we want to be usable as
  possible parents. It takes care of getting the relevant window handle
  (depending on the running platform) and stores it in a given pointer,
  either immediately or after a callback once the widget is mapped. So
  it can be used at construction. Also it sets a handle for X11 or
  Wayland.

In plug-ins:

- Screenshot uses the new gimp_progress_get_window_handle() directly now
  in its X11 code path and creates out of it a GdkWindows itself with
  gdk_x11_window_foreign_new_for_display().

Our inter-process transient implementation only worked for X11, and with
this commit, it works for Wayland too.

There is code for Windows but it is currently disabled as it apparently
hangs (there is a comment in-code which links to this old report:
https://bugzilla.gnome.org/show_bug.cgi?id=359538). NikcDC tested
yesterday with re-enabling the code and said they experienced a freeze.
;-(

Finally there is no infrastructure yet to make this work on macOS and
apparently there is no implementation of window handle in GDK for macOS
that I could find. I'm not sure if macOS doesn't have this concept of
setting transient on another processus's window or GDK is simply lacking
the implementation.