Improvements in the Context class

The TransformPoints and Distances methods are now
implemented in Context, so it is not necessary to provide
an implementation in the backend.

Improved the Distance struct.
[Mac] Remove obsolete methods in Context class
Add some initial infrastructure for the drawing tests
Reorganize test directory structure
Some more testing infrastructure work
Allow saving images
More work on drawing test infrastructure

Added first tests
[Gtk] Partial workaround for embedding an NSView inside a ScrolledWindow

Manually reposition the NSView when the enclosing viewport is scrolled.
Unfortunately, the NSView is still not clipped properly.

This is only meant to be a stopgap until the gtk stack is fixed,
at which point this commit should be reverted.
[WPF] Implement some missing image handling methods
[Xwt] Revert BackgroundColor set to white in Table.

This was introduced to workaround a problem in WPF but is no breaking the rest of the world.
[Xwt.Gtk] Fix bug where image created with .WithSize/WithBoxSize wouldn't be sized correctly in an Image widget
Fix Table layout issue
Update Xwt.WPF/Xwt.WPFBackend/ImageViewBackend.cs

Fixed typo.
Merge pull request #151 from residuum/patch-1

Update Xwt.WPF/Xwt.WPFBackend/ImageViewBackend.cs
[Xwt.Mac] Track MonoMac API break
[Xwt.Mac] Due to the monomac API change, we now have to specify base to avoid infinite looping
Add drawing tests
[Mac] Initial dialog backend implementation
[Xwt.Gtk] Put in a workaround for broken GdkEvent handling in gtk-sharp

Null GdkEvent objects are supposed to be marshalled to IntPtr.Zero
but the gtk-sharp binding generator generates the code as:
'event.Handle' instead of 'event == null ? IntPtr.Zero : event.Handle'

This is a trivial workaround for the problem which has no ill effects.
[Xwt] Add a simple Slider widget with a Gtk backend