Here is the thing. I know statuses are important but to be honest, I have still no idea how to implement them in order to keep everybody satisfied with implementation.
If you want simple statuses, I suggest just create a custom field. I just made it possible to create a custom field which can look like a drop-down list.
You can define list of drop-down options (e.g. statuses). Then select option which is applicable on individual invoices.
You can do the same on quotes, orders etc.
More advanced status implementation will most likely come after I implement warehousing and job-costing as these both concepts are somewhat related to statuses.