Show create folder/file only when a user can create them

[jelly]

This issue is only in limited mode as a administrator can create everything they want. To verify if we can create a file or folder in the current directory we can execute test -w instead of us re-implementing access and the trouble of checking ACL's.

Our options are:

• on contextmenu and kebab menu, test -w $CWD and cache the results based on current cwdInfo, so hashing it.
• in app.jsx on fsinfo.change() initially discover if we can write to the directory, if cwdInfo changes re-check.

Both approaches have one big issue, when an admin changes ACL's on a directory we do get an fsinfo change but the state lacks ACL information so cache invalidation would be difficult (as mtime does not change). So fsinfo would have to also encode ACL information in it's state.