Display overlay screen for blocking operations (#186)

* Catch state error in homepage widget

* Add flutter_overlay_loader lib

- Displays an overlay screen to indicate blocking operation

* Wrap blocking widget transitions in a loading overlay

- Prevents user from doing other things while loading
- Shows the user that something is happening

* Linting fixes

* Show overlay when uploading attachment file

* Show overlay when downloading file also

* Show overlay when loading or submitting API forms

- Major improvements to usability "feel"

* UI improvements for stock item test results widget

* Fix API_FORM bug

- onSuccess function was not being called

lib/widget/paginator.dart

@@ -384,8 +384,8 @@ abstract class PaginatedSearchState<T extends PaginatedSearchWidget> extends Sta
               PagedSliverList.separated(
                 pagingController: _pagingController,
                 builderDelegate: PagedChildBuilderDelegate<InvenTreeModel>(
-                    itemBuilder: (context, item, index) {
-                      return buildItem(context, item);
+                    itemBuilder: (ctx, item, index) {
+                      return buildItem(ctx, item);
                     },
                     noItemsFoundIndicatorBuilder: (context) {
                       return NoResultsWidget(noResultsText);
@@ -450,9 +450,11 @@ class NoResultsWidget extends StatelessWidget {
   Widget build(BuildContext context) {
 
     return ListTile(
-      title: Text(L10().noResults),
-      subtitle: Text(description),
-      leading: FaIcon(FontAwesomeIcons.exclamationCircle),
+      title: Text(
+        description,
+        style: TextStyle(fontStyle: FontStyle.italic),
+      ),
+      leading: FaIcon(FontAwesomeIcons.exclamationCircle, color: COLOR_WARNING),
     );
   }