communicate season selected event from SeasonsListFragment to activity

Create interface SeasonsListFragment.OnSeasonSelectedListener which
contains function onSeasonSelected(int seasonNumber). Make ShowActivity
implement this interface, and make SeasonsListFragment call its
attached activity's onSeasonSelected when a season is selected.
remove accidental setBackgroundColor
display list of episodes in a season when a season is selected.

Create new activity SeasonActivity, and start it when a season is
selected in ShowActivity. SeasonActivity contains an instance of the
new fragment EpisodesListFragment, which displays all episodes from
the specified season in a list.
remove empty widget from seasons list fragment