MacViews: Implement Tab Dragging

MacViews: Implement Tab Dragging

Looks like it's actually not too hard to *improve* on Cocoa and
actually allow the window to be dragged between spaces straight from a tab.
We'll see.

BUG=

[Lof, 2016-02-18 11:05:14]

lof84@yandex-team.ru changed reviewers:
+ lof84@yandex-team.ru

[Lof, 2016-02-18 11:05:14]

Hi tapted!

Why isn't this CL completed?
Is there any serious problems? Or just need some polishing?

[tapted, 2016-02-18 11:16:56]

On 2016/02/18 11:05:14, Lof wrote:
> Hi tapted!
> 
> Why isn't this CL completed?
> Is there any serious problems? Or just need some polishing?

There's an issue with the cursor potentially "slipping off" the tab at the
moment. dragging a tab at the top of a tab requires the cursor to move some
distance before starting a drag, and if the cursor point is off the tab at that
stage, then this won't function correctly.

In a way it's a problem for all platforms. The part of the tab should be the
point that the user first clicked, rather than where it ended after moving some
distance to begin the drag, and I started exploring a fix for that (which would
also fix MacViews) in https://codereview.chromium.org/1272223005/

It also needs some tests :/

[themblsha, 2016-02-18 15:34:32]

On 2016/02/18 11:16:56, tapted wrote:
> It also needs some tests :/

I added tests for tab drag&drop in Yandex.Browser, and I think I can adapt them
to the MacViews. Would be a nice opportunity to upstream some helper functions
that are sending CG events directly.