Adds Track-Only App Support (Addresses #119 and Sets Groundwork for #44) (#123)

- All Sources now have a "Track-Only" option that will prevent Obtainium from looking for APKs (though the App must still have a release of some kind so that a version string can be grabbed).
    - These Apps cannot be installed through Obtainium, but update notifications will still be sent.
    - The user needs to manually mark them as updated when appropriate.
    - This addresses issue #119.
    - It also partially addresses #44 by allowing some sources to be configured as "Track-Only"-only. The first such source (APKMirror) will be added later.
- Includes various UI changes to accommodate the above change.
- Also makes App loading a bit more responsive (sending Obtainium to the background then returning will now cause App re-load to pick up changes in App versioning that may have been made in the meantime, for instance through update checking).

lib/pages/import_export.dart

@@ -42,7 +42,7 @@ class _ImportExportPageState extends State<ImportExportPage> {
 
     Future<List<List<String>>> addApps(List<String> urls) async {
       await settingsProvider.getInstallPermission();
-      List<dynamic> results = await sourceProvider.getApps(urls,
+      List<dynamic> results = await sourceProvider.getAppsByURLNaive(urls,
           ignoreUrls: appsProvider.apps.values.map((e) => e.app.url).toList());
       List<App> apps = results[0];
       Map<String, dynamic> errorsMap = results[1];