Update L2TP to NetworkManager-l2tp 1.8.0 features

Summary:
- Update to NetworkManager-l2tp 1.8.0 features which include:
  - NetworkManager-l2tp 1.8.0 user certificate support.
  - Machine certificate support.
  - Rename Gateway ID to Remote ID.
  - Phase 1 & Phase 2 Lifetime TimeEdits.
  - Use IP Compression CheckBox.
  - Disable PFS CheckBox.
- Synchronize nm-l2tp-service.h with NetworkManager-l2tp 1.8.0's nm-service-defines.h.
- Base new user certificate UI on openvpn UI.
- Delete l2tpauth.ui and replace with programmatically generated  l2tpauthwidget.
- Add user and machine private key / certificate password support to l2tpauthwidget.
- Detect if libreswan or strongswan is being used and change UI based on which one is detected, e.g. disable "Disable PFS" check box with strongswan, Phase 1 & Phase 2 Lifetime show default strongswan and libreswan values when unchecked.
- Disable IPSec settings button if libreswan or strongswan not detected.
- Remove visibility of PPP Authentication group box if user certificate authentication is selected.
- Support loading *SWAN Base64 encoded PSK, but for backwards compatibility use unencoded for saving settings.
- For backwards compatibility with older NetworkManager-l2tp use `NM_L2TP_KEY_IPSEC_GATEWAY_ID` instead of `NM_L2TP_KEY_IPSEC_REMOTE_ID` for saving connection settings.

Reviewers: jgrulich

Reviewed By: jgrulich

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27764