macosx:Introduce VLCMainWindowCollectionViewItem cocoa class with XIB

This introduction is for making libraryView based on NSCollectionView by default for VLC 4.0
The following files are newly introduced
 - VLCMainWindowCollectionViewItem.xib : An Interface for showing collectionView items.
 - VLCMainWindowCollectionViewItem.h   : Header file for defining instance and instance variables
 - VLCMainWindowCollectionViewItem.m   : Cococa class which contains implementation of instance and representation of objects
-  Build VLCMainWindowCollectionViewItem files