Tabs on bottom of the titlebar WebExtension

This extension puts the tabs at the bottom.

How to install in Firefox v63+ Nightly or DevEdition:

1. Open about:config?filter=extensions.legacy.enabled and set it to true. This is a WebExtension but the API is still rough around the edges, it needs some legacy stuff.
2. Open about:debugging and load the manifest.json from disk.

Unfortunatelly this cannot be signed by Mozilla and published on AMO until this issue gets fixed.

The CSS is based on Timvde/UserChrome-Tweaks/tabs-on-bottom-titlebar.css:

More info about the new theme_experiment API on the addons blog.