Misc. Improvements

These are just some misc improvements that I think exaile could use.

* Move podcasts out of the radio tab, they do not relate to the radio.
* Make all playlists use the queue manager interface, and remove the queue manager tab. It serves no purpose that I can see, as it's just a playlist without the automatic start of playing the music, and with buttons.
* Look at bmpx for improvements to podcasts
* Add icons to menu objects
* Remove the plugin install/upgrade functionality. All plugins are installed, they are just not enabled. Installing would make sense to install an external plugin.
* Track Information should popup an error if no song is selected.
* Change New Playlist and Export Playlist menu options to File -> Playlist -> New/Import/Export
* Create a nice theme