(Bug 2774) (Bug 4892) (Bug 820) Alt text for insert image popup

* option to add alt text from HTML editor's insert image popup
* i18n text explaining alt and linking to faq in HTML and RTE popups using new nonfree faq hook
* adds some new aria roles for forms, dialogs: role alert, role dialog, aria-label
* put modal in front of tab order: works in ie, chrome, safari, ff. not opera. auto-focus to dialog.
* on webkit, the auto-focus highlights a pre-existing bug where the iframe is a wee bit longer than the content (you can see the white outline on any browser in prod). Webkit browsers highlight the popupiframe when it's focused, which makes the bug more obvious.