bug#41343: tab-bar-mode: Close tab on mouse-2 click

[Juri Linkov]

> I have tested it on GNU/Linux and macOS, and it seems to work fine.
>
> One concern is that tabs close on mouse down, rather than release.
>
> In Firefox, tabs close instead on mouse release.  This is good,
> because it gives me a second to react and move the mouse cursor away
> in case I misclick.
>
> This is currently less important in Emacs, as the buffer survives even
> such a misclick.  But maybe in the future users will want tabs to be
> more strongly connected to their associated buffers, and work would
> then easily be lost to a misclick.
>
> Could we change this to close on mouse release instead?

It will be easy to implement closing on mouse release.
There are also many other small adjustments planned.
But first I'd like to merge the basic completed work.
Then to continue tweaking in master.