Implement cell id based on source

[lexnederbragt]

Closes #213.

• created cell ID based on hash of 'source' field
• avoids conflicts by adding a running number to IDs for cells with identical 'source' fields in the same notebook

Tested my own projects and my demo project, and seems to be working fine.

[lexnederbragt]

Duplicate of #216 and can thus probably be ignored.

[lexnederbragt]

Note that this implementation avoids conflicts when cells have identical 'source' field content [edited:] by adding a running number to the ID rather than making each such cell have a unique ID.

[lexnederbragt]

The solution as implemented in #216 leads to a change in cell IDs if the order of cells changes, or for all subsequent cells if another cell is added in between.

This new version of the PR modifies the implementation for the IDs to ensure identical source fields always yields the same ID regardless of order, while at the same time keeping the solution to the problem of non-unique IDs, i.e. adding a running number '_1', '_2', '_3' ... to IDs with identical source fields.