plug-ins: set parent_window parameter to Screenshot function of portal…

… org.freedesktop.portal.Desktop (X11 only).
If we don't set this parameter appropriately, the created dialog is not
modal/transient relatively to the main GIMP window. This is especially
annoying because in my tests (GNOME shell), the screenshot dialog would
appear behind GIMP window, which makes it feel like broken behavior.

Note that the supported handles are X11 only so far. We'll need to
generate the appropriate Wayland handles too (created from xdg_foreign
protocol).

plug-ins/screenshot/screenshot-freedesktop.c

@@ -28,6 +28,10 @@
 #include <libgimp/gimp.h>
 #include <libgimp/gimpui.h>
 
+#ifdef GDK_WINDOWING_X11
+#include <gdk/gdkx.h>
+#endif
+
 #include "screenshot.h"
 #include "screenshot-freedesktop.h"
 
@@ -126,7 +130,24 @@ screenshot_freedesktop_shoot (ScreenshotValues  *shootvals,
                               GError           **error)
 {
   GVariant *retval;
-  gchar    *opath = NULL;
+  gchar    *opath         = NULL;
+  gchar    *parent_window = NULL;
+
+#ifdef GDK_WINDOWING_X11
+  if (GDK_IS_X11_DISPLAY (gdk_display_get_default ()))
+    {
+      GdkWindow *window;
+
+      window = gimp_ui_get_progress_window ();
+      if (window)
+        {
+          gint id;
+
+          id = GDK_WINDOW_XID (window);
+          parent_window = g_strdup_printf ("x11:0x%x", id);
+        }
+    }
+#endif
 
   if (shootvals->shoot_type != SHOOT_ROOT)
     {
@@ -138,9 +159,10 @@ screenshot_freedesktop_shoot (ScreenshotValues  *shootvals,
     screenshot_delay (shootvals->screenshot_delay);
 
   retval = g_dbus_proxy_call_sync (proxy, "Screenshot",
-                                   g_variant_new ("(sa{sv})", "", NULL),
+                                   g_variant_new ("(sa{sv})", parent_window ? parent_window : "", NULL),
                                    G_DBUS_CALL_FLAGS_NONE,
                                    -1, NULL, error);
+  g_free (parent_window);
   g_object_unref (proxy);
   proxy = NULL;
   if (retval)