Improve keyboard accessibility for multiple Quill instances

[kmoe]

When there are multiple Quill instances on a page (for example, a form that needs to be filled in with rich text data) and you have disabled tab using the technique in #110, it takes about 6 hits of the tab key to get from one field to the next.

This is due to the extra keyboard-focusable elements in the paste manager and tooltip modules.

Solution: add tabindex="-1" to these elements so they are skipped when tabbing through the page.

This also solves numerous problems with screenreaders not automatically reading down the page.

[jhchen]

I think making the paste-manager not tab-able makes sense. The tooltips both have a template so they're already customizable so it's not clear they need to have this markup by default as well. There should be a push for better accessibility and interoperability with screenreaders but that should be a separate issue where aria attributes are also properly set.

[kmoe]

Sure, I'll change this PR so it only sets the tabindex on paste-manager, and do some more investigation on screen reader issues (the app I'm using Quill in aims to be WCAG 2.0 compliant so I'll need to solve this problem at some point).

[kmoe]

Done.