Fluid 5439: Add docs for "Tutorial - Creating a New Adjuster UI" and panels in prefs framework.

@acheetham, this pull request is to replace the previous one in order to get rid of the trace of binary image files from the commit history. Those image files were added for "font icon research" page that we have decided not to convert.

This pull request has addressed all your comments for the previous pull: https://github.com/fluid-project/infusion-docs/pull/14

Ready for another review.