Cover fix segfault

the raise command won't work unless you set the correct parent. This means that dlgtrackinfo and dlgcoverartfullsize need to have the same parent. If they differ the cover will show under the trackinfo dialog. 

The segfault itself was caused because Qt destroys child widgets for us. Because I now give dlgtrackinfo a parent we don't need to delete it by hand anymore. In fact if we do that it causes a segfault because we are trying to delete an object that doesn't exist anymore.