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/github.dart

@@ -11,9 +11,9 @@ class GitHub extends AppSource {
   GitHub() {
     host = 'github.com';
 
-    additionalDataDefaults = ['true', 'true', ''];
+    additionalSourceAppSpecificDefaults = ['true', 'true', ''];
 
-    moreSourceSettingsFormItems = [
+    additionalSourceSpecificSettingFormItems = [
       GeneratedFormItem(
           label: 'GitHub Personal Access Token (Increases Rate Limit)',
           id: 'github-creds',
@@ -51,7 +51,7 @@ class GitHub extends AppSource {
           ])
     ];
 
-    additionalDataFormItems = [
+    additionalSourceAppSpecificFormItems = [
       [
         GeneratedFormItem(label: 'Include prereleases', type: FormItemType.bool)
       ],
@@ -96,8 +96,8 @@ class GitHub extends AppSource {
   Future<String> getCredentialPrefixIfAny() async {
     SettingsProvider settingsProvider = SettingsProvider();
     await settingsProvider.initializeSettings();
-    String? creds =
-        settingsProvider.getSettingString(moreSourceSettingsFormItems[0].id);
+    String? creds = settingsProvider
+        .getSettingString(additionalSourceSpecificSettingFormItems[0].id);
     return creds != null && creds.isNotEmpty ? '$creds@' : '';
   }
 
@@ -155,14 +155,11 @@ class GitHub extends AppSource {
       if (targetRelease == null) {
         throw NoReleasesError();
       }
-      if ((targetRelease['apkUrls'] as List<String>).isEmpty) {
-        throw NoAPKError();
-      }
       String? version = targetRelease['tag_name'];
       if (version == null) {
         throw NoVersionError();
       }
-      return APKDetails(version, targetRelease['apkUrls']);
+      return APKDetails(version, targetRelease['apkUrls'] as List<String>);
     } else {
       rateLimitErrorCheck(res);
       throw getObtainiumHttpError(res);