Need the ability to declare an attribute field as read only

[woodbri]

It seems like follow use cases would indicate that there is a need to be able to declare an attribute column as read only. Doing so would allow it to be displayed in panels for information, but would filter it out of any update requests.

Use cases:

1. gid unique feature id. it should never be set or changed, but it is useful to display it in case someone need to take action on an object outside of the GP application.
2. we have fields like shape_length, shape_area, that are computed values based on database triggers and should not be changeable.

I see two potential ways of approaching this problem.

1. implement ACLs on attribute columns (I think this is in another ticket)
2. implement this as a simple declare and filter

I'm sure there are other approaches that might be equally valid.

[adube]

Attributes handling is more and more done using custom forms and grids, so unless this becomes a need again I propose to set this one as "wontfix". For now, "Future".