[ADF-2676] Demo shell pagination bug fix (#3174)

* Demo shell pagination bug fix

* tslint error fix

* user preferences service fix

* refactor

* refactor

* refactor

* unit tests added

* unit tests fix

* [ADF-2676] added documentation for app config onChange stream

* [ADF-2676] renamed onChange stream to onLoad stream

* [ADF-2676] fixed test

docs/core/app-config.service.md

@@ -98,3 +98,28 @@ The supported variables are:
 | ------------- | ------------- |
 | hostname | `location.hostname` |
 | port | `location.port` |
+
+## App Config onLoad Stream
+When the app config is loaded correctly an onChange event is sent with the whole set app config properties. This comes in handy when a component wants to react to some property change or interact with the app config when it's correctly loaded.
+
+```ts
+
+    appConfig.onLoad.subscribe((appConfig) => {
+        console.log(appConfig); //this is the representation of the app-config
+    });
+```
+
+We have added also the `select` method where the user can give the property name which wants to be notified the when the app config is loaded and get the value.
+
+```json
+    appconfig : {
+        logLevel : 'trace'
+    }
+```
+
+```ts
+    
+    appConfig.select('logLevel').subscribe((logLevelValue) => {
+        console.log(logLevelValue); //this will be 'trace';
+    });
+```