Saving - Feature

Define the file format and define if zip it for the potential synch.

File should manage :
- Layers
- Brushes
- Palette

Format should be open and allow to add new features to come.

The issue names no files, tests, or entry points. First inspect the existing project structure to find how saving is currently handled, then define an open format covering layers, brushes, and palette data, with room for future features and a decision about optional zip-based synchronization.

Written by the indexing model from the issue text.