UI updates for NetworkManager-l2tp 1.2.6

Summary:
Could the plasma-nm/vpn/l2tp/ source be updated to support IPsec changes introduced with NetworkManager-l2tp version 1.2.6.

A screen shot of the current GNOME IPsec UI can be viewed here:
[[ https://github.com/nm-l2tp/network-manager-l2tp/wiki/Screenshots#ipsec-options | https://github.com/nm-l2tp/network-manager-l2tp/wiki/Screenshots#ipsec-options ]]

A summary of the changes for NetworkManager-l2tp 1.2.6 include:

Added the following to IPsec UI:
  - Phase 1 algorithms
  - Phase 2 algorithms
  - Enforce UDP encapsulation

Removed the following from IPsec UI:
  - Group Name

Copied nm-l2tp-service.h from NetworkManager-l2tp 1.2.6

Phase 1 & 2 algorithms UI additions are compatible with NetworkManager-l2tp versions 1.2.6 and 1.0.6 (from the 1.0.x series which has the changes backported from 1.2.6)

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6111