[Examples] Set a predefined cursor on Key.Space

Custom cursor are not enabled for OpenTK 1.1.2. They will be part of
OpenTK 1.1.3.
[OpenTK] Disable custom MouseCursors

They require further development, due to byte-order differences between
platforms.

MouseCursor is now derived from WindowIcon, which we will use for
INativeWindow.Icon support in the future, in order to remove the
dependency to System.Drawing.
[OpenTK] Treat null as MouseCursor.Empty
[Mac] Implement INativeWindow.Cursor