make notification states immutable

[hduelme]

Ensuring that notification states are immutable helps prevent any unwanted or accidental modifications.

[rcalixte]

Can you expand on the change here a little bit? What are the differences in the before/after in the user experience? Are there any examples that might be useful?

[hduelme]

Currently anyone using the API (for example spices) could accidental modify the stats, for example MessageTray.Urgency.CRITICAL = 0; could accidental be executed, resulting in a wrong internal state for CRITICAL. This change prevents this. Any changes made to this would be ignored, as cinnamon don't run in strict mode. For the end-user nothing should change at all.