Revert "Removed gtk-sharp download"
Revert "Removed gtk-sharp download"

This reverts commit ba752280433108da5cd205d6d8f97c9610483653.

wieslawsoltes/AvaloniaBehaviors,XamlBehaviors/XamlBehaviors,XamlBehaviors/XamlBehaviors,wieslawsoltes/AvaloniaBehaviors