After export-cancel, hide busy and cancel buttons

This is consistent with 70a71a82 (which is about the code path
through clicking on the 'x' status bar icon).

In the next commit I'll make a function to centralize this sequence
of killing a script and emitting the scriptKilled() signal.

src/frontends/qt/GuiView.cpp

@@ -4449,6 +4449,7 @@ void GuiView::dispatch(FuncRequest const & cmd, DispatchResult & dr)
 		}
 		case LFUN_EXPORT_CANCEL: {
 			Systemcall::killscript();
+			Q_EMIT scriptKilled();
 			break;
 		}
 		case LFUN_BUFFER_SWITCH: {