job-order-list-view: Expand rows

Expand the rows instead of setting min-width.
purchase-order-editor: Update header title when refnum changes
invoice-editor: Update header title when refnum changes
purchase-order-list-view: Don't dim refnum
invoice-list-view: Don't dim refnum
purchase-order-editor: Add margins to each invoice

This also rewrites the way invoices are added to make it more
maintainable.
purchase-order-editor: Update Remove button sensitivity

If there is no selected row, make the Remove button insensitive.
purchase-order-editor: Update selected invoice after add/remove
purchase-order-editor: Add scrollbar on invoice list

Adds a vertical scrollbar to the invoice list and shows it if there are
more than three items.
purchase-order-editor: Add placeholder to invoice list

Show "No invoices selected" if there are no invoices in the list.
job-order-list-view: Update list when search mode is toggled

Clears the list when search mode is enabled, and loads all items to a
new list when search mode is disabled.