macosx: Make library window freely resizeable

Constraining the library window to a video's aspect ratio is overly restrictive and makes interacting with the window difficult as window sizing becomes unpredictable when taking into account hiding/reappearing toolbars, playlist sidebars and so on. Like with the Qt library window, it makes more sense to make the library window freely resizeable.

The aspect ratio fitting window code has been moved to a new VLCAspectRatioRetainingVideoWindow class which is now what is subclassed by VLCDetachedVideoWindow, so that this can still resize itself to fit the video's aspect ratio.

Signed-off-by: Claudio Cambra developer@claudiocambra.com