remote-desktop/session: Add support for SelectionTransfer/Write

When a transfer request is done to the MetaSelectionSourceRemote source,
it's translated to a SelectionTransfer signal, which the remote desktop
server is supposed to respond to with SelectionWrite.

A timeout (set to 15 seconds) is added to handle too long timeouts,
which cancels the transfer request.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1552>

src/core/meta-selection-source-remote.h

@@ -30,6 +30,13 @@ G_DECLARE_FINAL_TYPE (MetaSelectionSourceRemote,
                       META, SELECTION_SOURCE_REMOTE,
                       MetaSelectionSource)
 
+void meta_selection_source_remote_complete_transfer (MetaSelectionSourceRemote *source_remote,
+                                                     int                        fd,
+                                                     GTask                     *task);
+
+void meta_selection_source_remote_cancel_transfer (MetaSelectionSourceRemote *source_remote,
+                                                   GTask                     *task);
+
 MetaSelectionSourceRemote * meta_selection_source_remote_new (MetaRemoteDesktopSession *session,
                                                               GList                    *mime_types);