Add ability to start lasso select within object without dragging layers

[hemu]

When beginning a lasso select from within an object, currently the behavior is to drag its layers. This PR updates this behavior such that a lasso select beginning within an object will remain a lasso selection over the existing layers.

NOTE: This is a simpler implementation compared to this alternative approach that includes a small refactor: https://github.com/GraphiteEditor/Graphite/pull/2799

Part of #2647

Before

https://github.com/user-attachments/assets/86142094-4e4d-4457-9f6c-241df25c3ad0

After

https://github.com/user-attachments/assets/64d97867-e38b-4699-8271-e51b4ba6af6f