Memory usage improvements

- Changed ImageToolsHelper to only load the GIF decoder
- Introduced custom Control ExtendedImageControl
ExtendedImageControl can display animated GIFs and other formats.
Animated GIFs will be loaded through ImageTools, everything else will
use the standard Silverlight methods.
- Speeded up image loading from cache
- Changed caching methods to accept the target image format (BitmapImage
or ExtendedImage)
- Changed ViewModels and Pages for compatibility with the above changes
- Reenabled image saving
- Changed image context menu, only static images can be saved