core: add is_connected interface

[julianoes]

This adds a sync interface for the connection status of a device. It is
basically the same information as also covered by the async methods
on_discover and on_timeout.

Closes #56.

[hamishwillee]

should be "Returns true if the specified device is currently connected."

[hamishwillee]

This allows you to test from a DroneCore object whether you have exactly one object connected or if a particular UUID is connected. It does what it says.

1. When/why would you want to check you have just one device connected?
2. I can imagine you might want to test whether a call to Device &device() will succeed but this API won't do that since it returns false if you have multiple connected devices.
3. The API to get the connection status for a particular UUID should be useful, because we could choose to not get handles to disconnected objects and monitor the connection API to start handling them as needed.

My problem with this is that I don't know if it allows us to sensibly handle the case where we disconnect and reconnect (both with 1 and multiple devices). We need to work out how we would do that in order to decide whether connection information needs to be testable from the Device itself, and whether we need to be able to register for connection/timeout events from the Device itself. Just FYI, added #99 for that example.

[julianoes]

1. When/why would you want to check you have just one device connected?

Whenever you only expect one device, so most of the time I'd say.

2. I can imagine you might want to test whether a call to Device &device() will succeed but this API won't do that since it returns false if you have multiple connected devices.

I agree that device() and is_connected() are not behaving the same way since device() returns the first device for the case of multiple connected devices. This makes me wonder if we should go back to the discussion whether device() should return a pointer which can be nullptr. That could make it consistent.

[hamishwillee]

When/why would you want to check you have just one device connected?

Whenever you only expect one device, so most of the time I'd say.

If I know I'm only ever going to get one device, test seems a little pointless.

[julianoes]

If I know I'm only ever going to get one device, test seems a little pointless.

Just to be sure and to know it is connected.

[julianoes]

I'm merging this for now. We can change it again later.

[hamishwillee]

OK. Thanks!