Make all Grafanalib objects immutable

[jml]

I would like to make all Grafanalib objects immutable, using attrs's frozen setting (see http://www.attrs.org/en/stable/examples.html#immutability).

Pros

• Easier to reason about
• Fewer bugs
• Reduced API support surface (don't have to worry about behaviour when people change things)

Cons

• People might be mutating things in their current dashboards, and this will break those changes.

I welcome input from existing users.

[matthewmrichter]

Is this still a concern?