Fix fileUsage test breaking behavior.

fileUsage tests were applying styles to the fixture element, which
apparently does not get cleaned between tests. This was because
fileUsage.$container had different semantics from element.$container
(widget's own container div vs. parent container div). FileUsage
now inherits from Element to make sure behavior is consistent.

Change-Id: I8fab8bcf084d8b7e480655114506d9848e9d9a49