jquery.makeCollapsible: clean up the handler toggling logic

Merged three functions which did almost the same thing into one,
togglingHandler(), which takes a few options instead.

Left the old functions as wrappers for now, I'll clean them up in
another commit.

Change-Id: Id3f457a84064382f2d962e9ac4d926827de93126
Merge "jquery.makeCollapsible: clean up the handler toggling logic"