TST-MiddleClick wot Qudeid

A very simple extension to TreeStyleTab (TST) to middle click reopen closed tabs found here:

https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/

Originally, this functionality was possible with the combination of TST and TabMixPlus. Since WebExtensions arrive in FF57, TST is now a sidebar but exposes APIs to further extend it's functionality.

This extension won't work without TreeStyleTab!