[WPF] Canvas: Clip the drawing area

Fixes issue when drawing outside the canvas bounds.
[Gtk] Fix Cairo pattern leak
Matrix: null-check Equals value
[WPF] Implement Scrollbar.ValueChanged

Based on a fix by Marko B.L.
[Mac] Fix Slider control orientation

Based on a fix by Marko B.L.
Merge pull request #275 from lytico/MatrixNullCheck140113

Matrix: null-check Equals value
Basic WebView added.
[Icons] Stock icons review
[Icons] Icon TODO review
[Icons] Icon TODO review
[Icons] Reverting my latest three commits
IWebViewBackend must extend IWidgetBackend.
Set the Url from the constructor.
WebView implementation for WPF.
Remove some unused methods from the Mac backend.
Merge pull request #278 from bratsche/webview

Webview
Added Xwt.Gtk.Mac and Xwt.Gtk.Windows support libs
[Gtk] Improvements in GtkPlatformBackend
Reorganized samples

Moved test app to a subdirectory.
Added sample app for XWT runnin on mac.
Fix reference to MonoMac