Introduce the StoreItem base class

[gycsaba96]

This is a followup to PR #1151 removing duplicated functionality. The main benefits include:

• reduced code duplication,
• isolated testing of common functionality,
• the possibility of a more targeted testing of the BaseStore class (which I would be happy to do).

The main changes are as follows:

• introduced a new class that implements common functionality for items in a BaseStore,
• added corresponding unit tests,
• fixed a typo in the copyright text.