Store the project model in a relational database

[dbarashev]

This is a summary issue for all tasks related to the storage of project model in a relational database

Overview

We plan to piggy-back on the relational database features, such as transactions, to implement real-time sync between the Desktop and Cloud, and merge offline Desktop data with the changes made in the Cloud. The first thing which needs to be done for this is storing and updating the project model in an in-memory project database.