Make the default Drawable constructor use non-multithreaded semantics, consistent with desktop platforms.
Make Drawable inherit from Container + implementation for Windows, Mac and Wpf. A stub implementation for Gtk.
Make Drawable inherit from Container + implementation for iOS.
Make Drawable inherit from Container: Fix for Winforms PixelLayoutHandler.
Make Drawable inherit from Container: Turn on WS_EX_COMPOSITED
Remove WS_EX_COMPOSITED flag that was added in the previous checkin.
Tab control test: added Add Tab button.