update refactoring of children() method

- is now find('children')
- test cases updated
added moveDown method & test cases
default `order by LFT ASC` when no customized order is found.

- dockblock updated
update dockblock
refactoring of test cases
update fo test cases. and some code refactoring

find('children') will throw when node was not found
more refactoring
even more
reverting the` Table::get()` thing, new test cases added
Merge pull request #3100 from quickapps/3.0-tree-behavior

3.0 tree behavior