Add support for RGB565

[ChristianFeldmann]

https://github.com/IENT/YUView/issues/620

Todo:

• [x] Rendering of single planes
• [x] detection of format from filename and file extensions. I think the '.rgb565' extension does not really exist. The default extension should be 'rgb' and then we should look for '565' or somthing similar in the filname.
• [ ] Differences
• [x] Big Endian 565