Improved cursor/focus ranges for screen readers while using Pencil Scratch ActiveCode

[sshumak]

Currently, when using the Pencil Scratch ActiveCode with a screen reader, it is possible for the cursor to leave the modal entirely and return to other parts of the screen (example attached). Additionally, when the modal is closed via the close button, the cursor doesn't automatically return to the pencil button that initially launches the modal. Addressing these issues would greatly improve the accessibility of the ActiveCode modal.
(The focus is currently on the "Source" button despite the modal being open)