Make the uses widget nicer.

Main changes include:
* Fix contexts not being clickable (they were clickable but didn't work)
* Remove the blue arrows; all widgets are expanded by default and can
be collapsed with a small link in the text instead. This operation is
not common enough to justify such a huge ugly button.
(The main problem with the button was that it looked ugly, tough)
* Contexts without actual uses are always sorted last.
* Add some nice icons to the list.
* Remove all the ugly frames.
* Make text displaying code use a fixed-width font.
* Make the tooltip use a fixed-width font too, make it smaller (just 2
lines of context instead of 3), and highlight the line with the use
by making it bold-face.
* Re-format and rename all the widget headers to be more readable
and look nicer.

REVIEW:107296