define rootClass to rule all items class

base  rootClass in options, add the additional class string to define
all other class.  make easier to display different style to
multi-sortable list on one page.
define rootClass to rule all items class

base  rootClass in options, add the additional class string to define
all other class.  make easier to display different style to
multi-sortable list on one page.
Add a new options: fixedDepth

Set ‘fixedDepth’ to true to force items only can drop it in  the same
depth position in list.    use for some list has specific type,
eg:'Volume,Section, Chapter ...'
Revert "Add a new options: fixedDepth"

This reverts commit 26a28cd2f78a424af762fabb22d6aeeeecfcd2b0.
Set  'fixedDepth' default value to false.

Set the default value of new option: 'fixedDepth'  to false.
Add a new option 'fixedDepth'

fixed item's depth, use for some list has specific type, eg:'Volume,
Section, Chapter ...'