Improvements Estimate and Invoices tools

[pekeso]

Fix:

• Update the .csv test files, currently they are all in a old format. It would be useful to keep the old format files and adding some cases with the new format.
• Check the "Helps" in the dialogs, currently the help links does not arrive on any useful pages.

Improvements:

• It would be nice when we import the invoices, to add the new contacts in the contacts table using the data available in the csv. Before adding the contacts, we should ask the user if he wants to add them, after the contacts are added in the table then we import the invoices.

[PloszajAaron]

A client warned us about two more bugs we need to fix:

• When importing Invoices, if the field "InvoiceDescription" is empty we get an error when we try to open the invoice's print preview. The same could happen also with the estimates.
• The German translation of the extension commands says two times "Rechnungen exportieren" (Export invoices), we should use "Offerten exportieren" as in german we call the application "Offerten und Rechnungen"