bluez5: implement Bluetooth MIDI nodes

Implement BLE MIDI node, providing an input and output port.  MIDI data
received is normalized and produced on output port.  Data from input
port is converted to BLE MIDI and sent to the device.

Event timestamps come from device clock, whose offset to wall clock we
determine by correlating packet reception times with their timestamps.
The jitter in packet reception is likely much larger than device clock
error, so we average over that.

spa/plugins/bluez5/meson.build

@@ -33,6 +33,7 @@ bluez5_sources = [
   'dbus-monitor.c',
   'midi-enum.c',
   'midi-parser.c',
+  'midi-node.c',
 ]
 
 bluez5_data = ['bluez-hardware.conf']