database: Add functions for manipulating invoices
job-order-list-view: Add column for invoice and payment
Add invoice list view widget
database: Add functions for mapping invoices with purchase orders
purchase-order-editor: Add a list box for mapped invoices
Update libgd submodule
database: Add functions for loading mappings to models
Add invoice editor widget
purchase-order-editor: Load invoices list
purchase-order-editor: Map invoice after list view is closed
WIP