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/api.dart

@@ -651,12 +651,6 @@ class InvenTreeAPI {
    */
   Future<void> downloadFile(String url, {bool openOnDownload = true}) async {
 
-    showSnackIcon(
-      L10().downloading,
-      icon: FontAwesomeIcons.download,
-      success: true
-    );
-
     // Find the local downlods directory
     final Directory dir = await getTemporaryDirectory();