Allow users to choose where to save a scene every time.

If a scene has not been saved before, and `Save` is used, or any time `Save as` or `Save a copy` is used, a file explorer dialog box should be opened to allow the user to choose where to save the scene.

This approach may benefit from a consolidation of the separate scene files into one file, as well as the creation of a separate `snapshots` directory within `.chunky` that contains subdirectories for each scene, each containing the snapshots.

Start by tracing the existing Save, Save as, and Save a copy entry points and how scene files and snapshots are currently stored. Review the proposed separate snapshots directory and scene subdirectories before deciding the scope. Done means each requested save action opens a file chooser when appropriate and preserves scenes and snapshots in the agreed layout.

Written by the indexing model from the issue text.