Make properties consistent

[niccokunzmann]

Properties should have a consistent interface when it comes to unexpected values.

Examples:

• Event.description
• Todo.summary
• Journal.start

For that, we need to check the property value type and possibly the return type that is contained in the properties they refer to.

Questions arise:

• What if a property is invalid/malformed - do we return None or do we raise an error?
• Do we expect a different behaviour for Event.START and event.start?

What is your input on that?