Group Features - Collapsible and Filterable

This implements two improvements for groups.

1) For the standard multiselect, I added a collapse / expand icon near each group. This causes the group to collapse, yet clicking on it in it's collapsed state will check / uncheck its children.
2) For the filter plugin, I added support to search by group name. If you type a group name, or part of it, all of its children will be displayed, even if the checkbox itself does not match.