added ui buttons to collapse/expand action.
added autoCollapse option so the menu can be initialize as collapsed or expanded (defaults to expanded so it acts like the original functionality).

needed changes:
clicking the group header need to select collapsed items, but doesn't.
filter doesn't seem to work correctly.
need to add option to disable collapse button (and default to it) so the multiselect acts like the original design.

niceities:
add a vertical line on the left so you can tell what group you are currently looking at.