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 https://github.com/SeaDve/Kooha/pull/264 first.

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