Don't try to show notification if background agent is disabled.

In WP8 this causes a crash.
Don't try to copy non-existent cover art to live tile.
Fix DB thread crashes. Use using() whenever DB is updated.

Still UI fixes needed to keep the UI state up to date.
More DB fixes.

- Make settings work for subscriptions
- Playback starts.
Fix DB: Make play progress work again.
Fix play state after restarting app.
Clean player code now that player cannot control episode state.
Fix saved play position.
Fix DB: Populate episodes again via a property.

This also seems to have a positive impact on the performance and cleaning episodes after settings changes seems to also work now.
DB fix: Make podcast subscription texts work again.
DB fix: Make deleting subscriptions work again.
Dix DB: Make settings work + make listened episode clean up work.
DB fixes: Make downloading progress in episode listing work again.

Also some other cleanups.
Fix DB: Make downloaded episodes list work again.
Fix DB: Make delete episodes / mark episodes refresh properly.
DB fix: Fix crashing when playing episodes.
Fix DB.
Fix DB: Fix performance issue with main list.

- We still need to update the subscriptions list when subscriptions change.
- We also need to fix unplayed/partially played texts.
- We also need to fix history view.
Fix DB: Performance improvements for the main view.
Fix DB: Update episode status after delete all/mark all