when clear ngSortable area not in ngSortableItem, clear all current select
add sortable shfit flag class
create get ShiftFlag class
add parent variable indicate last modify item by shift key
add helper function for remove shiftFlagClass
add first shift flag element. If user not press shfit key, the shift flag will replace by the newest key
remove shift selection class when clearing all children select method
handle shift key behavior
update source code