Built in function PlayMedia does not play music folders

This is a pull request to backport https://github.com/xbmc/xbmc/pull/2965 into Frodo

Following specification, PlayMedia should be able of playing content of
folders, either with content of video or music.

An issue has been found by which passing a music folder to this
function won't play its content. However, if the folder contains video
files it works.

This function internally enumerates files by extension, and when a
folder is received, only video extensions are searched. This commit add
music extensions too to the search, solving the issue.