Improve usability of "Add content" dialog from the keyboard

This is a follow-up to #2155377: Only show one widget preview at a time when adding content in panels where we created the "Single" preview mode for the "Add content" dialog.

Currently, tabbing through that dialog requires a lot of tabbing because you tab over both the preview link and the inline "Add" button for each widget. So, if there are 10 items and you want the 10th, you have to press tab 20 times.

Here is the proposal worked out with @cboyden to improve this:

1. We'll take the inline "Add" button out of the tab order (by hiding it with display: none rather than opacity: 0)
2. When the preview is rendered, rather than having the focus end up on the inline "Add" button, have focus on the "Add" button that sits above the preview fieldset
3. Move the "Add" button that's above the preview fieldset into the fieldset legend, and make it look more like a standard button

#3 is important, because without that, it's not necessarily clear that focus has moved to that button. This is because users won't necessary have the opportunity to see the button before it's focused, and it doesn't doesn't really look like a button (this is the only time in the entire site that a button looks like a green tab!).

So, after these changes, you'll only tab over the preview links, and then hit enter once to generate the preview, and enter again to add the widget.