tabindex first pass

Hi Cam,

I put in the first pass in on tabindex.

- It is cross-linked in the attribute index.
- Each element refers to it in the intro section called "navigation
attributes" https://svgwg.org/svg2-draft/intro.html#TermNavigationAttribute
This will provide a place for us to add SVG Tiny directional navigation
attributes later
- You will find tabindex in the Interactivity section under Focus and then
Sequential Focus Navigation and tabindex. The Section on Sequential focus
navigation is designed to be in line (almost verbatim) with HTML5. I still
need to address some of the links as you will see in the Issue comments.
Some of the definitions like browser context refer directly to the
definition in the HTML5 TR spec. as we agreed on the calls.
- I also have not addressed the follow up on the Document Object and
deprecation of SVG parity events to those in HTML and replacement with HTML
events.

It is a good start. Dominic Mazzoni, at Google, asked me to let him know
when something landed in the SVG editors draft. I shared this with him
today he said he would get with the Google SVG folks to get work started on
it.

Rich