Remember and retreive last filter setting based off section key.

Added two functions to library.js: setFilter and getFilter.

The filter is set in the respective sections and retrieved in the top level index.js and passed on to the section which already knows how to display it.

The default category when no filter is found (like in the case of initial startup) is "all"

Tested on 2012 TV.  Please let me know if this is at all useful.

Cheers.