jquery.makeCollapsible: events for collapsing/expanding, tests

jquery.makeCollapsible: events for collapsing/expanding, tests

Trigger a custom event to allow callers to hook to the
collapsing/expanding, allowing the module to be testable and making
it possible to e.g. implement persistence via cookies.

Added tests for the triggers themselves and for some basic
asynchronous collapsing functionality.

The following four events are fired (the namespace used is the same
one as the one used for firing the click event for initial toggle):
* beforeExpand.mw-collapsible
* beforeCollapse.mw-collapsible
* afterExpand.mw-collapsible
* afterCollapse.mw-collapsible

Change-Id: I580365770d061418a6734fcb54b2d86f2a821748