Fix heading updating in toolbar plugin
More generic way of getting buttons

This works for both oerpub and github-book, by using the first button in
the "headings" area as the heading button rather than a specifically marked
item. Also, fix small bug that assumed the element in the range is the heading
itself.
Remove last reference to headingFunc
Merge pull request #23 from oerpub/oerpub-heading-plugin

Fix heading updating in toolbar plugin.
Tested on OERPUB client and OERPUB's github-book successfully. Should be working for CNX' github-book editor as well.