[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
[Xwt] Fix last commit broken bits
Convert the drag image to a bitmap
ListViewColumnCollection Add methods now return the column object.
Added an IsResizeable property to ListViewColumns
Added coordinate calculation functions to text layout.
Added support for text layout attributes.
Added overload of GetIndexFromCoordinates that takes a point.
Merge branch 'textcoordinates'

Conflicts:
	Xwt.Gtk/Xwt.CairoBackend/CairoTextLayoutBackendHandler.cs
	Xwt.Gtk/Xwt.GtkBackend/TextLayoutBackendHandler.cs
	Xwt.Mac/Xwt.Mac/TextLayoutBackendHandler.cs
	Xwt.WPF/Xwt.WPFBackend/TextLayoutBackendHandler.cs
	Xwt/Xwt.Backends/TextLayoutBackendHandler.cs
	Xwt/Xwt.Drawing/TextLayout.cs
Implemented text layout memory management & pango byte index mapping.
Conflicts:
	Xwt.Gtk/Xwt.CairoBackend/CairoTextLayoutBackendHandler.cs
	Xwt.Gtk/Xwt.GtkBackend/TextLayoutBackendHandler.cs
	Xwt.Mac/Xwt.Mac/TextLayoutBackendHandler.cs
	Xwt.WPF/Xwt.WPFBackend/TextLayoutBackendHandler.cs
	Xwt/Xwt.Backends/TextLayoutBackendHandler.cs
	Xwt/Xwt.Drawing/TextLayout.cs