Merge pull request #180 from BnLG/nx70

Gigaset, add support for N670 IP PRO

Hi,
Let's add support for the new Gigaset N670 IP PRO (and also for the Gigaset N870E IP PRO), which shares exactly the same configuration as the already existing N870 IP PRO.
Let's then create a Nx70 IP PRO plugin (which is how Gigaset names them).
First commit is then a 1:1 copy of N870-83.v2.48.0 plugin.
Next commits are modifications, explained below.
It add support for N670 IP PRO and  N870E IP PRO.
It updates firmware to last 83.v2.49.1 version.
It enables SIP TLS (kept disabled in #147), which now properly works (SipProvider.0.UseSIPS must be set to 0, after support from Gigaset team).
It properly sets ToneScheme according to configured locale.
It deregisters accounts for detached handsets (WithDeregOnHsNotReachable), which makes more sense than the contrary.
Finally, it renames the keys to the ones defined in the README.
Thank you 👍
Note that this is a rework of #159, after the move to Python3.

Reviewed-by: Alexandre Fournier <None>