Also show power level bluetooth headphones

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**

I love how the indicator shows me the power levels of my bluetooth devices.
But, it never includes my bluetooth headphones/headsets even when they are also connected:

**Describe the solution you'd like**
I'd love for these devices to also be included.

**Existing work**
Android is able to read out their power levels:

**Describe alternatives you've considered**

**Additional context**
Looking through the code it depends on dbus & Upower. Looking there I see that the bluetooth devices aren't listed.

Some devices like the MX Keys also seem to have a `org.bluez.battery1` interface

While the headphones don't have that interface:

Relevant Stack overflow question with some interesting answers:
https://stackoverflow.com/a/52899481/1544200

https://github.com/wangrunz/BluetoothMonitor#battery-level-for-bluetooth-headsets-support-handsfree-profile

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