Middle click add and close tab

[lathan]

Currently, middle clicking in the tab area does nothing.

This would do two things:

* Middle click in tab area adds a new tab (almost identical to double_click)

* Middle click on an existing tab closes it.

One example of this functionality can be seen in Firefox. I've written a patch for the first, it works but I'm not 100% sure it's the best way to write it (I iz n00b). Will look into the second one later.

[lathan]

middle click in tab area adds new tab

[lathan]

Something is not right with this patch. When I double middle-click I get a total of three tabs opened - 1 tab for the initial click and 2 tabs for the second click.

[Dokan]

 https://github.com/Guake/guake/issues/67