PLUGIN: Multiple - undo_push changes location in the code unexpectedly

[Danweel]

CAREFULLY standardize the way the undo push works.

1. Sometimes the undo_push is inside the logic function, sometimes it's in the operator class.
2. Push the undo state ONCE at the end of the execute method with a clear message description

This is just for compliance and best practices. If it doesn't work well, this can be wontfixed.