Using callgrind it was very visible that amarok is spending an enormous amount of time to turn podcast episodes into tracks.

Initially i noticed that the PlaylistBrowserModel is calling Playlist::tracks().count() while it should be calling Playlist::trackCount().
Constructing a list of tracks is expensive for the case of Podcasts (as internally podcast episodes are converted into tracks which
get added to a new list). This change already improved the situation a bit, but the UI was still sluggish. Callgrind was still showing
that we are turning episodes into tracks - over and over.

To fix that i added a cache to the PodcastChannel. As long as the episodes haven't changed, there is no point in constructing
a new list of tracks each time tracks() is called.

With that second fix the UI is now pretty useable.

Tested with http://www.justing.com.cn/justpod/justpod_ch.xml