Make `categoryMultiSelectOptionType.tpl` behave like `categoryOptionList.tpl`

Make `categoryMultiSelectOptionType.tpl` behave like `categoryOptionList.tpl`

Currently the `categoryMultiSelectOptionType.tpl` outputs nodes of depth 0 and depth 1,
but article categories for example can be nested deeper than that.
The `categoryOptionList.tpl` as used by the category add forms handles arbitrary nesting levels already,
so I’ve ported the template logic over to the multi select option template.

The maximum nesting depth in the `AbstractCategoryMultiSelectOptionType.class.php` has been changed
to the default value (-1) to allow for infinite nesting depths.