[Feature Request] Merge title-bar and tabs bar

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• iTerm2 version: 3.0.20161212
• OS version: macOS Sierra 10.12.2 (16C67)

Please note that this is a Feature Request, not a bug report.

This issue follows from #5255. Is it possible to merge/overlay the tabs-bar on top of the titlebar? I made this mock to explain what I mean:

There can be a small space between the close-minimize-zoom and tab bar which can be used to drag the window around. This is mostly inspired from the Unity Convergent terminal [1].

Thanks!

[1] http://design.canonical.com/wp-content/uploads/4b17/terminal-blog-desktop-detail2.jpg

Kind of a dup of #292 (closed) but I think I went off in the wrong direction with that issue and it's worth another try using the existing tab control. It is time for an updated look.

changed milestone to %Review for next version

Me myself find this implementation "prettier" than the chrome-style tabs from issue #292 (closed), space is much more optimized. Maybe using the background color for inactive tabs and a lighter or darker color for the active one

I've implemented a prototype and gif! In https://github.com/gnachman/iTerm2/pull/318

@prashnts I think this could be a good way to go