Restructure Tk's stub library: No longer use Tcl_SetResult() for setting the error message,
but Tcl_ResetResult/Tcl_AppendResult, as all other stub libraries do. This will allow us
to remove Tcl_SetResult() in Tcl 9.0, eventually. More structural improvements, taken
over from Tcl 8.6's tclOOStubLib.c/tclTomMathStubLib.c and from Tk 8.6's tclStubLib.c