Fix deselect and select option when using options with empty values
Add an option to build optgroups with additional checkboxes and click all events
Fixup to https://github.com/ovgu-ttz/bootstrap-multiselect/commit/a383df6d2f4543113c918ed6c272a9899af58ddf so optgroup checkboxes have proper formatting
Fix 'Select All' when there are dividers