- ItemListFragment.refreshList now takes a filter string as a parameter, and saves the new filter to member variable _filter if it is not null
- remove member variable _filter from MainActivity - the fragment should keep track of that
- onFilterSelected() should pass the filter string to the ItemFragment (via refreshFilter())
- add hard coded decade filters to getAllKeywords()
- in getAllShowingSongsCursor() check to see if a decade filter was selected.  If so, modify the SQL query accordingly

Change-Id: I5e185c6116e41ba4687b84c659fa6a1ad30d81f6