Add a preview-size option to track the current window size

[blerner]

I've previously been using Tab Preview to see thumbnails of background tabs, but since it's not compatible with e10s, I've switched to TabScope. One feature I particularly liked about Tab Preview was its preview size was tethered to the current Firefox window size, which ensured that every preview had the same aspect ratio as the window. This in turn meant that when I switched to that tab, if my eye had picked out a key part of the page in the thumbnail, I'd know where proportionally in the page to look for that key part. TabScope currently only has the ability to specify a fixed size for the thumbnail, which means that any eye-blink intuition about where-key-parts-of-the-page-are doesn't necessarily carry over.

Tab Preview has a few other options about what to show, which would be great to support: