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/l10n/app_en.arb

@@ -333,6 +333,12 @@
   "filterTemplateDetail": "Show template parts",
   "@filterTemplateDetail": {},
 
+  "filterTrackable": "Trackable",
+  "@filterTrackable": {},
+
+  "filterTrackableDetail": "Show trackable parts",
+  "@filterTrackableDetail": {},
+
   "filterVirtual": "Virtual",
   "@filterVirtual": {},