name of toolkit (e.g. "tcltk", "RGtk2", "Qt" (not qtbase)). If NULL, then we search for it in a) an iherited toolkit object b) the "guiToolkit" option (which can be set via options("guiToolkit"="RGtk2"), say. If that fails, we prompt for a selection for any installed toolkit. In the typical usage, this all happens in the background, except perhaps once.

In design this is to allow different toolkits to be used with different GUIs, but due to differences in event loops, this often leads to lockups, so is not recommended.