added missing French translation string
added initial German translation
use getter instead of undocumented magic property
increase the backend socket timeout to 5 seconds like it was initially
define common watch button options in the base class
refactor the way a media item is turned into a list of playlist items

An item can now act as a Streamable which provides functionality for
returning an array of ItemLink objects. The PlaylistItem constructor
has been modified to take an ItemLink as parameter and deduce all
necessary information from it. This enables us to reuse all of the
playlist-related code, and it makes it easier to add functionality
for creating playlists for complete TV shows.
move the "file" property to StreamableTrait since it's not actually
available for all Media items
add ability to download a playlist for a complete TV show (fixes #179)
updated CHANGELOG.md