feat: allow choosing audio device

[SeaDve]

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

[SeaDve]

I think this will be hard to integrate with the current UI, without having #264 first.

The future plan is to merge this PR to that PR.