Fix #343, add the poss^Cility to use the test backend (WIP), improve testability of OsmConnection (WIP)

State.ts

@@ -98,6 +98,7 @@ export default class State {
     public readonly featureSwitchIsTesting: UIEventSource<boolean>;
     public readonly featureSwitchIsDebugging: UIEventSource<boolean>;
     public readonly featureSwitchShowAllQuestions: UIEventSource<boolean>;
+    public readonly featureSwitchApiURL: UIEventSource<string>;
 
 
     /**
@@ -201,7 +202,6 @@ export default class State {
             this.featureSwitchShowAllQuestions = featSw("fs-all-questions", (layoutToUse) => layoutToUse?.enableShowAllQuestions ?? false,
                 "Always show all questions");
 
-
             this.featureSwitchIsTesting = QueryParameters.GetQueryParameter("test", "false",
                 "If true, 'dryrun' mode is activated. The app will behave as normal, except that changes to OSM will be printed onto the console instead of actually uploaded to osm.org")
                 .map(str => str === "true", [], b => "" + b);
@@ -209,6 +209,10 @@ export default class State {
             this.featureSwitchIsDebugging = QueryParameters.GetQueryParameter("debug","false",
                 "If true, shows some extra debugging help such as all the available tags on every object")
                 .map(str => str === "true", [], b => "" + b)
+
+            this.featureSwitchApiURL = QueryParameters.GetQueryParameter("backend","https://openstreetmap.org",
+                "The OSM backend to use - can be used to redirect mapcomplete to a testing backend or e.g. openHistoricalMap")
+
         }
 
 
@@ -217,7 +221,9 @@ export default class State {
             QueryParameters.GetQueryParameter("oauth_token", undefined,
                 "Used to complete the login"),
             layoutToUse?.id,
-            true
+            true,
+            // @ts-ignore
+            this.featureSwitchApiURL.data
         );