a2Item_Editor - make multi_item_editor from Hotstrings implementation

[ewerybody]

Turns out there is more need for such an item-editor widget with multiple named dictionaries to edit.
The Hotstrings-implementation already HAS everything for that like:

• creating new dictionary items, name check, ...
• selection fills editor accordingly
• renaming of dicts
• deletion
• moving selections between dictionaries

• move things to ui/a2widget/a2item_editor.py
• amend demo