Accounting Interface; added web service interface (for future Xero development)

The Accounting Interface has been updated for future development of web based API services such as Xero.

Global Settings, General group, GL and Accounting page has been updated as follows:

• Export Type is now mandatory if "Use General Ledger?" is checked.
• The layout of controls has also be revised.
• New "Xero" Export Type.
• Uses a Private Application and therefore requires a Public/ Private Key. For more information see https://developer.xero.com/documentation/api-guides/create-publicprivate-key.
• Verify option displays information regarding the Organisation, Contacts and Tracking Categories.
• Note, this is not yet complete and should only be used for testing purposes. It cannot be used to actually create Bank Transactions and Manual Journals.

Each Entity, if licensed for the Entities module, can override the global Xero settings.

GL Accounts, Import All drilldown now supports importing General Ledger Accounts from Xero.

Note, the Xero API interface is incomplete and is ALPHA release only.