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/main.dart

@@ -154,6 +154,7 @@ class _ObtainiumState extends State<Obtainium> {
               0,
               ['true'],
               null,
+              false,
               false)
         ]);
       }