Default account number format for clients & suppliers

[spl1nes]

Create a way to define default client & supplier account numbers. This not only includes the format but also the number range (e.g. clients start at 100000 and suppliers at 700000).

File: Modules/Accounting/Admin/Installer.php

This should probably get defined in the application settings. This means that per unit we probably need 2 settings. And a default format and number range should be defined during the installation of the Accounting module.