Cover netbook

@cardinot @kain88-de

This branch contains some performance improvements, tested on my netbook. 
As reported before there was a performance problem even if no covers where displayed. 
It turn out that his was called by the "update()" calls which unnecessarily invalidates the whole track table. 
After replacing this with better calls, I have adopted the "CachedOnly" approach for WCoverArt as well.

This results in a behavior, that the cached covers are painted without a delay. All un-cached covers will appear after 100 ms at a line. This works at the netbook with a lot of waveform stutter, but at least in a controllable way. 



```// it updates cover column``` ?