version 1.2.10 release

Compatible with NetworkManager 1.2.0 and later.

• Point version 1.2.10 appdata image URIs to nm-1-2 github branch:
  • https://raw.githubusercontent.com/nm-l2tp/network-manager-l2tp/nm-1-2/appdata/l2tp.png
  • https://raw.githubusercontent.com/nm-l2tp/network-manager-l2tp/nm-1-2/appdata/l2tp-ppp.png
• Corrected force UDP encapsulation toggle button behavior.
• Workaround for libreswan ipsec status issue with short (< 8 char) PSKs.
• fix gcc -Wimplicit-fallthrough warning.