Various little changes.

*) rename Playlist::Model::saveM3U to Playlist::Model::savePlaylist to reflect what it does
*) Make inserting items into the playlist with options & Append not start playing them, even if nothing is playing. This caused unpleasant things to occur on startup.
*) remove the --engine command line option.
*) Initialize the sidebar in init() along with everything else
*) Fix a crash that could occur when quitting amarok with no track playing.
*) Remove unnecessary includes
*) remove the --enqueue command line option. This one has been deprecated for a really long time.