Fix #338 Pulse plugin does not work on freshly installed Freeseer

Do not set the device if there is none set in config.

Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Fix #342 Mixer source inputs are reset on config load

Connections need to be enabled after configuration is loaded otherwise
the connections will cause the configuration to be set to the first
entry in case of comboboxes.

Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>
Fix #339 USB plugin resets input source when returning to settings dialog