There is no obvious choice what works best here, but this is what
TigerVNC has been doing for years without complaints. Let's follow
them until we get reports that this doesn't work well.
It seems Apple has fixed their buggy keyup events, so remove the
workaround and allow keys to be kept pressed again.
This is a revert of 9e99ce126ca8f6f350fa015c0e58d35103c62f7e.
The standards have unfortunatly caused some confusion between the Windows
key and the original Meta key. Try to handle the common case sanely at least.
The Firefox workaround which checks for missing Alt key events may
synthesise new KeyboardEvents. On these events, checkAlt should not be
recursively triggered. Otherwise, we get "too much recursion" errors
whenever the Alt key is pressed.
