Change .panel's position from absolute to fixed.

It will be more usable with long files.

Here's two test cases:

* http://dxr.mozilla.org/mozilla-central/source/js/src/jsdate.h
* http://dxr.mozilla.org/mozilla-central/source/js/src/jsdate.cpp
Limit the nav panel to just short of the height of the viewport, and give it a scrollbar when it needs one.
Pin the nav panel. Close #244. Ref bug 968020.