Fixes #227

TODO

• Save the last selected device on the settings
• Properly handle if there is no default device (use a custom row that becomes No Devices, instead of adding None on the list)
• Adjust volume
• See current volume level (via a GtkLevelBar or something)

FIXME

• Use GstDevice.display_name instead of our own
• Use GstDevice.reconfigure_element instead of creating an element and getting its device property