Fix oversized album cover art

When viewing the "Full Size" album covers from the cover
manager or the now playing widget, if the cover was higher in
resolution than that of the monitor, it would get cut off. Also,
the window was resizeable however the image was not. This patch makes
the dialog a fixed size and resizes the cover only if it is too large to
fit on the screen both horizontally or vertically. It also now displays
the dimensions of the original artwork in the title bar.