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
Implemented utf-8 mapping & all indices are now int instead of some
beeing uint.
Fixed drawing with gtk text backend.
Fixed text decorations
Changed GetExtendsFromIndex to GetCoordinateFromIndex.
Cleaner API & easier to implement in the mac backend
Changed the text attribute api.
Removed unused text attributes.
Merge pull request #158 from mkrueger/master

Text attributes & coordinate transformation