Added option for local playback

This is a solution for issue #31

Long press on a item will show the option "Launch local viewer":
*  remote/local music items will launch the default music player and play it
*  remote/local video items will launch the default video player and play it
*  remote pictures will NOT work, not supported using android apps
*  local pictures will launch the default picture viewer

Long press on a directory result in a toast: "No action available"