Add a data storage class

[vkbo]

Epic: #3
Related: #4

The reading and writing of the project's JSON files should be done via a storage class. The path to each file within a project can then be handled internally in such a class. Since everything is JSON, the loading of both files and individual values can be standardised to avoid duplication of code.

The benefit if having a single class that handles all file I/O is that it would be easy to extend such a class to allow storing the project data in a zip archive, or switch between folder and zip storage with a flag.