Cover art support dlgtrackinfo

This fixes two issues that jus mentioned a while back. 

the coverart widget will now show the last displayed cover until it gets a response from the covercache. In return the cover cache will always send a signal that a pixmap is found defaulting to the default cover. With this guarantee wcoverart does not need to care about pixmaps that were not found.

The other issue is that cover changes in dlgtrackinfo could not be canceled like other metadata changes. This required a bit more cleaning up and refactoring a long the way but I explained everything in the commit messages. 
This comment was added because of a Daniel's request.