4.0.0.3-release-1

1. MP4 video not saved has been resolved, reported by webvid mb and
LaruYan.
2. Drag n Drop now supported in Subtitle and Attachment.
3. Subtitle and Attachment only valid for MKV, prompt an error message
if user choose MP4.
4. Now window size and full screen can be resume, if user safely exit.
5. Readme and Build script improved!