refactor(@desktop/general): connection to undefined prop fixed

An error `ReferenceError: mainModule is not defined` is fixed.
That started to happen when we moved registered context properties
from constructors to `load` proc, because of lazy context property
inatialization (after we ensure that a user is logged in), but main.qml is
loaded when the app start, when we don't have `mainModule` registered yet.

ui/main.qml

@@ -161,9 +161,11 @@ StatusWindow {
     }
 
     Connections {
-        target: mainModule
-        onOpenStoreToKeychainPopup: {
-            storeToKeychainConfirmationPopup.open()
+        target: startupModule
+        onAppStateChanged: {
+            mainModule.openStoreToKeychainPopup.connect(function(){
+                storeToKeychainConfirmationPopup.open()
+            })
         }
     }