Added placeholder for source plugin to read from an existing mkv file.
Added corresponding worker placeholder to transcode an mkv file using
ffmpeg.
Updated the sources page to show which sources come from which plugins.
The config class now supports a type column in the database which
specifies how the value should be interpreted. The class defines a
number of scalar types, which are returned without transformation. A
string list type is also defined, which splits the value into an array
based on newlines.
Also added phpdoc comments to all class constants, members and methods.
