[GREENFOOT-459] Add 'Cmd-m' shortcut on Mac to minimise window
'Cmd-m' on Mac to minimise a window is a system-wide convention. We should support it (on all top level windows).
This means to change the shortcut for 'Insert method' in the Java editor (which I think is Cmd-m currently).
- Issue type: Task
- Priority: Low
- Fix versions: 3.7.0