Turn left panel buttons into toggles

Even though the left panel has a close button, some users expect the buttons that show the panel to also hide the panel (particularly when there is only one button).

This change makes the Legend, Detail and Editor Buttons act like toggles to hide/show
the left panel.  Try it at http://rawgit.com/regan-sarwas/basic-viewer-template/ToggleButtons/index.html

This was tested with a map that had just a legend as well as a map that had a legend and details.  It was *NOT* tested with the edit button.