mainwindow: set the updating flag later for sinks and sources

The updating flag is used to determine whether a change in some UI control state is due to a user action or an update from the server. The flag was set to false too early when processing sink and source updates from the server. This caused at least unnecessary "set port" commands, because each update refreshes the port list, which generates change events in the port list combo box, causing the portChanged callbacks to be called.

The unnecessary "set port" commands could even confuse the automatic port selection policies in the server. If the current port wasn't originally explicitly chosen by the user, then the "set port" command from pavucontrol will make the server think that the user explicitly chose that port.