support list, expandable context menu

- list creates a child tree element per item
  - the key is a numerated index
  - context menu on the item deletes this exact item
- context menu handling now separated on which options are available and
their execution

Bugs to fix:
- Type of Dict not correct (see example "cascaded_list")
- Keys of dict not editable when in list (e.g. type
List<Dict<something>>)

next things to do:
- converters for dict, list, intenum (partially just check)
- tuple & pair
- sanity checks after checking rest of converters