Teach the CodeHacker to override field write accesses

Needed for the prefsDir handling about to be ported over from
fiji-compat's IJHacker into ij-legacy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ij-legacy (Windows): do not store the preferences in the current directory

It is almost never what the user wants.

Ported over from fiji-compat.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>