wayland: emit GDK_SELECTION_CLEAR on owner changes

The wayland backend currently never emits GDK_SELECTION_CLEAR events.
GtkClipboard uses this signal in order to clear the clipboard owner when
the selection is set to something outside the application.
This commit ensures the wayland backend emits GDK_SELECTION_CLEAR before
setting the clipboard owner to NULL, as this means we lost the
selection.
Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>

https://bugzilla.gnome.org/show_bug.cgi?id=790031

gdk/wayland/gdkselection-wayland.c

@@ -113,6 +113,7 @@ struct _GdkWaylandSelection
 
 static void selection_buffer_read (SelectionBuffer *buffer);
 static void async_write_data_write (AsyncWriteData *write_data);
+static void emit_selection_clear (GdkDisplay *display, GdkAtom selection);
 
 static void
 selection_buffer_notify (SelectionBuffer *buffer)
@@ -921,6 +922,7 @@ data_source_cancelled (void                  *data,
   if (context)
     gdk_drag_context_cancel (context, GDK_DRAG_CANCEL_ERROR);
 
+  emit_selection_clear (display, atom);
   gdk_selection_owner_set (NULL, atom, GDK_CURRENT_TIME, TRUE);
   gdk_wayland_selection_unset_data_source (display, atom);
 }
@@ -1031,6 +1033,7 @@ primary_source_cancelled (void                                *data,
   display = gdk_display_get_default ();
 
   atom = atoms[ATOM_PRIMARY];
+  emit_selection_clear (display, atom);
   gdk_selection_owner_set (NULL, atom, GDK_CURRENT_TIME, TRUE);
   gdk_wayland_selection_unset_data_source (display, atom);
 }
@@ -1272,6 +1275,27 @@ emit_empty_selection_notify (GdkWindow *requestor,
   gdk_event_free (event);
 }
 
+static void
+emit_selection_clear (GdkDisplay *display,
+                      GdkAtom     selection)
+{
+  GdkEvent *event;
+  GdkWindow *window;
+
+  event = gdk_event_new (GDK_SELECTION_CLEAR);
+  event->selection.selection = selection;
+  event->selection.time = GDK_CURRENT_TIME;
+
+  window = _gdk_wayland_display_get_selection_owner (display, selection);
+  if (window != NULL) {
+    event->selection.window = g_object_ref (window);
+    event->selection.requestor = g_object_ref (window);
+  }
+
+  gdk_event_put (event);
+  gdk_event_free (event);
+}
+
 void
 _gdk_wayland_display_convert_selection (GdkDisplay *display,
                                         GdkWindow  *requestor,