database: Add methods for loading purchase orders
Add purchase order list view widget
Add purchase order editor widget
job-order-editor: Add field for purchase order

This will create a new page listing the purchase orders.
purchase-order-list-view: Push an editor to create/edit

Push an editor when the New button is clicked or an item is selected.
purchase-order-list-view: Allow close only when in selection mode

Allow closing only when the view is in selection mode and there is an item
selected.
purchase-order-list-view: Add method for getting the selected item
job-order-editor: Get purchase order when list view is closed

Get the id and reference number of the selected item and update the fields
when the list view for purchase orders is closed.
Add egg-list-box submodule
Link to egglistbox
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