Add tree view for project notes

[vkbo]

Summary:

Generalise the item, model and tree classes and add the needed functionality to load multiple project trees.

Related Issue(s):

Resolves #40

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• Function descriptions are complete and understandable
• Only files that have been actively changed are committed