Checkbox group (parent/children) functionality. Makes a checkbox parent of other checkboxes.
If the parent is clicked the children are checked/unchecked based on the parent.
If all the children are checked, parent gets checked too.
If there is an unchecked item in the children and if the parent is checked, it gets unchecked.
children can be a function. In this case, that function is run agaist the parent in order to find the children. This is useful for groups of checkboxes.
Checks/unchecks parent based on the initial status of its children at start.
