utils: add cache.go
graphic: refactor code, move GenerateCacheFilePath() to utils
gdkpixbuf: refacetor code, about Blur() and BlurImage()
graphic: refactor code, about resize.go, add ResizePrefer()
graphic: refactor code, about ImplXXX() functions
gdkpixbuf: refacetor code, fix function names and documents
graphic: move blur.go to gdkpixbuf and fix test code
gdkpixbuf: fix Blur() issue and add test code
graphic: refactor test code
graphic: update ClipImage(), change arguments
graphic: refactor code, rename ResizeXXX() to ScaleXXX()
graphic: add ScaleImagePrefer()
gdkpixbuf: add more functions about image file and complete test code
graphic: remove FileScale and FillPreferScale
gdkpixbuf: add GetPreferScaleClipRect()
gdkpixbuf: add ScaleImageCache() and ScaleImagePreferCache()
gdkpixbuf: refacetor code, about ConvertImageToXpixmap()