[PAINTROID-811] Save project on back

[Knist1704]

Save as project is now prompted when returning from workspace with unsaved changes in the canvas.
Jira Link 811

Refactorings and Bug Fixes

• Added a Project object to WorkspaceState
• Returning from the workspace now shows a save dialog when there are unsaved changes
  • Overwrites the existing project when editing a saved project
  • Saves as a new project when creating a new image
• Saving as an image no longer updates the command history

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