Provide a toolkit-agnostic interface for "busy indicators", which
allows the 3D viewer to show a busy cursor, without the canvases
having to to know how a wxCursore works.
The motivation here is to decouple the 3D renderers from the WX
GUI system, as they can then be used when when there is not an
active window (e.g. for offscreen rendering).
Otherwise, attempting to use a wxBusyCursor without a GUI
available is an instant segfault.
John Beard