CoverArtCache - passing pixmap in the signal/queue by pixmapFound(...)

@kain88-de:
Since we have a timer now can we just go back to the first signal/queue model that passed the images around. It is much easier to understand and the code that follows from it is also easier to understand.
WCoverArt - connect to pixmapFound using Qt::DirectConnection

@kain88-de
If we make the signals of type Qt::DirectConnection then Qt will process that signal immediately and not store copies in an event-queue.
Merge branch 'coverArtSupport' of https://github.com/cardinot/mixxx into coverArtSupport_2
Adpting branch to the latest merge - getting pixmap directly from signal pixmapFound(..)
dlgtrackinfo - removing unused pixmap member
Rename default cover art - using default_cover.png instead of vinyl-record.png
Getting default cover location from CoverCache + fixing warning boxes
it avoids starting filebrowser in default_cover location (when user wanna change the cover)
WCoverArt - avoids loading default_cover as real cover