Make connect() return a device handle

[jyasskin]

#2 made requestDevice not imply a connection, but that implies that common uses look like:

navigator.bluetooth.requestDevice(...)
.then(device => {device.connect().then(()=>device);})
.then(device => { /* real use */ });

If connect() returns a BluetoothDevice or similar, then the common use would look like:

navigator.bluetooth.requestDevice(...)
.then(device => device.connect())
.then(device => { /* real use */ });

which is a bit simpler.