Basic MythTV plugin configuration

Plugin support in MythTV is pretty good, but a little rough around the edges.

1. Add a button to the mythtv project.

In my case, I added the following block to [mythtv/programs/mythfrontend/library.xml]:

  <button>
     <type>DJ_PLAY</type>
     <text>Start DJ</text>
     <action>JUMP Start DJ</action>
     <depends>mythmusic</depends>
     <depends>mythdj</depends>
  </button>