Created a struct representing a Playlist instead of using a tuple.

Also moved the Playlist to libdmp-common, where it needs to be.
Warning fixes, mostly signed vs unsigned fixes.

But appearantly the Qt5 cmake module system can still be circomvented. So I also added isystem to the cmake includes which in turn allows me to reenable the no default case in switch warning.
Cleanup the playlist model code with some more boost fusion magic.
Removed some raw loops and replaced them by algorithms and boost fusion magic.