TabFern as a sidebar (TFaaSb?)

Nobody has claimed this yet.

It makes a lot of sense, and should work nicely if we do a Firefox port. However, [Chrome does not support sidebars](https://bugs.chromium.org/p/chromium/issues/detail?id=477424). If anyone knows how to use a content script to drop the current page into an `iframe` without trashing the history and the Back button, please let me know :) . [This example](https://github.com/curtislacy/inject-sidebar) overlays the sidebar (I think), which strikes me as suboptimal.

However, it is possible to get the size of the current monitor by creating a full-screen window. That means we can:
- Get the screen size
- Position the TabFern window on the left/right (at user option). (What width? Current width? Percentage or other dimension at user option?)
- Position all of the other Chrome windows (on that monitor?) in the remaining space
- Make sure the window that was on top still is!

Requested by a user on 2017/11/05.

Start by reviewing the content-script and iframe approach, including the linked inject-sidebar example, then investigate the proposed full-screen window and Chrome window-positioning behavior. Done would require an agreed sidebar or window-management design, including width, monitor placement, remaining-window layout, and preservation of the previously focused window.

Written by the indexing model from the issue text.