Main screen loading indicator (#183)

* Bug fix for login screen

- Prevent setState() from being called if the widget is no longer loaded

* Add callback function when API status changes

- Home screen uses this function to update connection status indicator

* Linting fixes

lib/l10n/app_en.arb

@@ -908,6 +908,9 @@
   "serverNotConnected": "Server not connected",
   "@serverNotConnected": {},
 
+  "serverNotSelected": "Server not selected",
+  "@serverNotSelected": {},
+
   "sounds": "Sounds",
   "@sounds": {},