Added SystemBackend class

To be used to implement desktop-specific functionality
Ongoing work to improve the Image API

An image can now have multiple versions with
different resolutions.
Ongoing work on Image API

Some adjustments in the image handler API.
Implement the Mac side of things.
[WPF] Track changes in the image API
Add some basic bitmap caching support
Merge branch 'master' into improved_images

Conflicts:
	Xwt/Xwt.csproj
Implemented WithAlpha in Image class

This method replaces the old ChangeOpacity method in
the BitmapImage class.