Improvements for the due date feature

[MohamadNateqi]

close #640

[alexmigf]

Just one remark. You have placed the method get_due_date() in the Invoice document class, but the display_due_date() on the main class. Since this is restricted to the Invoice, don't you think they should both be in the Invoice class?

[MohamadNateqi]

It seems that the invoice class is being called multiple times. If we were to move the display_due_date() function to this class, it would add a due date to the invoice every time it's called.
Initially, I tried this approach, but I noticed that multiple due dates were being added to the invoice, which made me realize that the class was being called multiple times. So, I moved the function back to the main class.

[alexmigf]

In that case I think we should make the function display_due_date a bit more flexible:

• adding a filter in the document type check
• using wcpdf_get_document instead of wcpdf_get_invoice
• add a callable check under get_due_date, just in case
• Renaming this hook: wpo_wcpdf_{document_slug}_due_date_display

[MohamadNateqi]

In that case I think we should make the function display_due_date a bit more flexible:

To make it more general purpose instead of just working for the invoice document. Got it.

[MohamadNateqi]

@alexmigf In the latest commit, I also updated the hook name for filters. Instead of adding the document slug in the hook name, I send the document type as an argument to make them uniform. It seemed there was no need to have different hook names for them.
Please let me know if it's not a good idea, and I'll revert that.

[MohamadNateqi]

@alexmigf Ready to review.

[alexmigf]

Perfect!