Xero Interface; updated validation and OAuth initialisation code

The Xero Accounting Interface has been updated as follows:

• Extra validation has been added to ensure the Xero interface cannot be used if the Global General Ledger Export Type setting is not "Xero".
• Previously if you had configured the Xero interface but changed to another export type you could still use the Xero interface via script code.
• OAuth initialisation code changed to accept the Client Id.
• Previously the OAuth initialisation for an Entity would always use the Global Settings Client Id.