JDK-8089986: Potential fix for beeping on windows.

[Dean Wookey]

Hi,

On Windows, a beep sound always plays when using accelerators/mnemonics
with the alt key, even if those accelerators exist. It's only supposed to
beep when no such key combination is registered.

I've found a solution which never beeps which I believe is preferable,
however I'm a bit out of my depth here.
https://github.com/DeanWookey/openjdk-jfx/commit/3f8520e22f5090a35097005254133f2eac8f97cc,
based on this stack overflow post:
https://stackoverflow.com/questions/3662192/disable-messagebeep-on-invalid-syskeypress

It appears like Javafx doesn't use the system menu at all? And hence we can
safely respond to the WM_MENUCHAR message by closing the system menu?

Dean