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/part_detail.dart

@@ -253,15 +253,17 @@ class _PartDisplayState extends RefreshableState<PartDetailWidget> {
             title: Text(L10().partCategory),
             subtitle: Text("${part.categoryName}"),
             leading: FaIcon(FontAwesomeIcons.sitemap, color: COLOR_CLICK),
-            onTap: () {
+            onTap: () async {
               if (part.categoryId > 0) {
-                InvenTreePartCategory().get(part.categoryId).then((var cat) {
 
-                  if (cat is InvenTreePartCategory) {
-                    Navigator.push(context, MaterialPageRoute(
-                        builder: (context) => CategoryDisplayWidget(cat)));
-                  }
-                });
+                showLoadingOverlay(context);
+                var cat = await InvenTreePartCategory().get(part.categoryId);
+                hideLoadingOverlay();
+
+                if (cat is InvenTreePartCategory) {
+                  Navigator.push(context, MaterialPageRoute(
+                      builder: (context) => CategoryDisplayWidget(cat)));
+                }
               }
             },
           )