Add ability to select default bluetooth audio output mode

When connecting my bluetooth headphones, it is always set to Headset mode, which makes it impossible to listen to music. One has to install `blueman` and choose the output as Audio Sink (H2DP) each time one connects, to get normal sound.

**Describe the solution you'd like**
A dropdown with "Default output-mode", which changes the mode on the device, and remembers it the next time it is connected.

Start by tracing how the Switchboard Sound Plug discovers and configures Bluetooth audio devices, then inspect the existing PulseAudio-related handling for output modes. Confirm how a selected default mode could be retained and reapplied on reconnection; done means the dropdown changes the device mode and the choice persists for later connections.

Written by the indexing model from the issue text.