Visually seperate the widget from the question editor

Summary:
This diff is a precursor for the minimizability of widgets. The gray is supposed to indicate the parts that won't ever be minimizable.

From the terrible pre-border-radius world of
{F13659}
to the post border-radius of
{F13655}

Even does answer-widget
{F13656}
(but not hints yet; that has its own problems)

Test Plan:
TIL: border-radius is supported in all modern browsers (and ignored in IE8)
1) Test that border-radius actually works in all browsers as expected.
2) See that the widgets are now separated (visually).

Commentary: the "visually" part of the diff is because the widgets divs are still descendants of questionArea/answerArea. Nothing has really changed here except the css.

Reviewers: alpert

Reviewed By: alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7040