[GREENFOOT-622] Editor key bindings cannot be changed

In the preferences, the Editor key bindings pane doesn't work correctly. If you choose an action and press "add key", then:

on linux, no key that you press seems to be detected
on Mac OS X, pressing a modifier key seems to be detected as a complete key. So for instance pressing "cmd" instantly sets the key as "cmd+unknown keycode (0x300)" (despite that a second key was not yet pressed).

Should check this for BlueJ 4.0 as well; not sure if it is also an issue there.

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