Sales order support (#438)

* Add new models for SalesOrder

- Create generic Order and OrderLine models with common functionality

* Refactor

- Move some widgets around
- Cleanup directory structure

* Add link to home screen and nav drawer

* Add SalesOrder list widget

* Linting fixes

* Fix string

* Refactor PurchaseOrderDetailWidget

* Tweaks to existing code

* linting

* Fixes for drawer widget

* Add "detail" page for SalesOrder

* Add more tiles to SalesOrder detail

* Allow editing of salesorder

* add list filters for sales orders

* Display list of line items

* Customer updates

- Display customer icon on home screen
- Fetch sales orders for customer detail page

* Cleanup company detail view

* Create new sales order from list

* Stricter typing for formFields method

* Create new PurchaseOrder and SalesOrder from company deatil

* Status code updates

- Add function for name comparison
- Remove hard-coded values

* Update view permission checks for home widget

* Add ability to manually add SalesOrderLineItem

* Add nice progress bar widgets

* Display detail view for sales order line item

* edit SalesOrderLineItem

* Fix unused import

* Hide "shipped items" tab

- Will be added in a future update

lib/preferences.dart

@@ -11,6 +11,7 @@ import "package:path/path.dart";
 // Settings key values
 const String INV_HOME_SHOW_SUBSCRIBED = "homeShowSubscribed";
 const String INV_HOME_SHOW_PO = "homeShowPo";
+const String INV_HOME_SHOW_SO = "homeShowSo";
 const String INV_HOME_SHOW_MANUFACTURERS = "homeShowManufacturers";
 const String INV_HOME_SHOW_CUSTOMERS = "homeShowCustomers";
 const String INV_HOME_SHOW_SUPPLIERS = "homeShowSuppliers";