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/app_sources/signal.dart

@@ -24,14 +24,12 @@ class Signal extends AppSource {
     if (res.statusCode == 200) {
       var json = jsonDecode(res.body);
       String? apkUrl = json['url'];
-      if (apkUrl == null) {
-        throw NoAPKError();
-      }
+      List<String> apkUrls = apkUrl == null ? [] : [apkUrl];
       String? version = json['versionName'];
       if (version == null) {
         throw NoVersionError();
       }
-      return APKDetails(version, [apkUrl]);
+      return APKDetails(version, apkUrls);
     } else {
       throw NoReleasesError();
     }