Update use of `FlushMenu` to `FlashMenu`

[morgant]

Now that the FlushMenu built-in function has been deprecated in favor of the corrected FlashMenu function in mlvwm, we should replace all occurrences of FlushMenu with FlashMenu. That said, while FlushMenu will still work, there's not yet an mlvwm release which includes the new FlashMenu, so we'll want to wait until that has been released & packaged.