[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.
Font fixes

Fixed methods that return the system default fonts.
Added font tests.
Implemented RichTextViewBackend for Mac
[Mac] Fix font creation
Merge pull request #176 from grendello/RichTextView_Mac

Implemented RichTextViewBackend for Mac
[WPF] Implement new Font api
[WPF] Font size conversion fixes

In WPF, font size is based in 72 dpi, while device pixels
are 96 dpi. XWT assumes that font and pixel dpis are
the same, so we need to make a conversion.
Add WPF Expander
Added Html clipboard format & fixed rich text format on windows (gtk
backend).
[Gtk] Show EventBoxes if we are using them to set the background color
Merge pull request #178 from samclarke/wpf-expander

Add WPF Expander
Merge pull request #179 from mkrueger/master

Added Html clipboard format/fixed some issues with RTF in the GTK backend.
Add some tests for the Color class