List enhancements and cleanup

I've done some enhancements and cleanup to the whole list loading in the Home-Activity and everything related (which I wanted to do since the API rewrite^^):

- to simplify the code all list loading is now done by ItemGridFragment, basically it was implemented twice as some lists were loaded by Home and some by ItemGridFragment
- moved the loading indicator from activity_home.xml to fragment_animelist.xml as now the indicator can be displayed independent for anime- and mangalist (they can have different loading times depending how many items are on them)
- keep selected list after rotation
- **IMHO most important improvement:** don't reload list after rotation: anime- and mangalist are now serializable so they can be passed to the Bundle in onSaveInstanceState and reloaded in onCreateView (this also keeps the scrolling position), this prevents unnecessary redownloading of lists (most popular, upcoming etc.) and makes the UI smoother
- some general cleanups and optimisations