Allow Base#_set() to exclude properties.
Allow Item#_changed() to function before #_project is set.
Move and unify handling of Item#_set(props) to new Item#_initialize(), and add support for props.insert = false.
Fix avoiding of name clashes in cloning.
Improve tests for moveAbove() / moveBelow().
Use non-deprecated versions.
Merge code for Item#insertAbove() / #insertBelow() and fix issue with wrong indices.

The previous code was faulty when item was not already in the children list.
Have #insertAbove/Below() return a boolean.
Improve documentation of insertion methods.
Switch to returning items instead of booleans in #insertAbove/Below() and adjust documentation accordingly.
Simplify code for #insertAbove/Below(), by introducing a private helper.

Making overriding in Layer simpler.
Fix Layer#insertAbove / insertBelow test.