Enhancements mainly for adding podasts

Changed default RSS icon. Subscribed podcasts are now removed from
listviews. Also changed player default RSS icon.
Fix Bug #17307 Podcast search NullPointerException
Fix Bug #17138 Filter does not work on rotation

Make EpisodeFilters serializable and store them if the activity is
recreated. Also update the UI elements that display the
filter status (e.g. the check box in the menu item if the
NewFilter is activiated). I also tried around to prefill
the search term box if a filter is active, because right
now the current search term is reset if you click on the
search icon. Even after a while of trying i could not find a
workardound for this - it seems like this is the way it should
be, i.e. if you click on search the old search data is wiped
and a new search starts.
Add callbacks to container statically

While I'm not sure this may be related to Bug #17272.
Fix Bug #15804 screen rotation test connection

Remove the old proprietary functionality and implement
a callback instead.
README.md: add build instructions