Initial work to support multi-resolution images
No more font size units
Improve support for multi-resolution images
Improvements in the bitmap generation api
Bitmap generation fixes
[Mac] Fixed image rendering
[WPF] Implement hi-res icon support
[Mac] Implement ToBitmap
[Mac] Fix image rendering
Merge branch 'retina'

Conflicts:
	Xwt.WPF/Xwt.WPFBackend/FontBackendHandler.cs
	Xwt.WPF/Xwt.WPFBackend/ImageHandler.cs
Improve text layout handling

TextLayout can now be created without a reference
to a canvas or context. Fixed drawing of text when
using an image builder. Added text drawing tests.
Fix some image conversion and font issues

Use the correct size when generating an image for a drag operation.
Use invariant culture when parsing font size. This value is
usually provided by the programmer, not the user, so it makes sense
to use invariant here.
Fix text layout initialization issue.
[Mac] Fix a sizing issue in ScrollView
Add some null checks
Add new api for loading toolkits
Don't reload a toolkit if it has already been loaded
Add 'unknown' value to ToolkitType
Remove Context.Font property

Text font is set in TextLayout.
Added Reset method to Context. It will be used to initialize
a context to a known initial state.
Track api changes and reorganize context backend implementation

Get rid of the cairo-based TextLayout backend. We use pango.
Added api for getting the system fonts

When using the GTK backend on windows or mac, 
the native backend will also be loaded.
Also removed obsolete text layout constructor.