the collections of utility class and function
DDView: subclass from NSView, which is used to accept dragging files.
DNPopupView: subclass from NSView, that display with a popup animation
DNMarqueeTextFiled: subclass from NSTextFiled, that display the long string with marquee animation.
DNLinkButton: subclass from NSButton to display web link
DNTrackButton: subclass from NSButton, can has 3 states(normal. mouse down and mouse in)
NSImage+CGImage: the NSImage category creat cgimage