Undo-Redo-Mechanism issues

**ArmorPaint version:**
ArmorPaint.org - v0.8 (2021-01-30) - 77d0ef2

**OS/device including version:**
Win64

**Issue description:**
The undo-redo mechanism does not work with layer groups as expected

**Steps to reproduce:**
First problem:
1. Create several layers
2. Changing their order and redoing it works perfectly fine
3. Now create a group, for xample
4. Redo it immediately and ArmorPaint switches into a unusable state

Second, related problem:
1. Reopen ArmorPaint
2. Create several Layers and groups
3. Assign Layers to groups
4. Redo some steps
5. The behavior is incorrect. Layers are shifted to wrong positions

No source file, test, or entry point is named. Reproduce the issue on Win64 using the listed layer-group and reopen/reorder steps, then trace the undo-redo handling for group creation and layer assignment. Done means redo no longer makes ArmorPaint unusable and layers retain their intended positions after reopening.

Written by the indexing model from the issue text.