jquery.makeCollapsible: cleanup

* Ensure correct handling of togglers which hook to multiple elements
  and elements which have multiple togglers
* Move the `options === undefined` check outside the loop
  per CR on I8dd4f0d0
* Get rid of the old private toggling functions for various kinds of
  toggles (obsoleted, but left alone, in Id3f457a8)
* Don't use separate logic for initial toggling of custom togglers
* Consistently use collapseText/expandText (uppercase T) instead of
  collapsetext/expandtext (lowercase t) where possible
* Adjust comments and whitespace

Change-Id: Id8ecd8f577fb4cd3f640a7830a85436a301b3e2c
Merge "jquery.makeCollapsible: cleanup"