Mac and WPF changes

Hi Vivek,

Here's a rundown of the major changes:

- Fix project files for Monodevelop (removes conditional statements)
- Mac: Should work for drawables inside a layer now (e.g. when a BackgroundColor is set)
- Drawable.CreateGraphics() is only supported by WinForms, GTK, and Mac, and added a new DirectGraphicsSection to test it
- Gtk: Drawable can now be used as a container
- Wpf: A bunch of commits I forgot to merge in for last pull request