Suppress obsolete warnings on Mono

[patriksvensson]

Mono seems to care about obsoleted aliases even though they only exist in generated code and isn't explicitly called. We could work around this by creating a custom attribute for aliases like CakeObsoletedAliasAttribute.

[devlead]

Could also be that we should ignore on the generated classes.

Also on that note, we should probably now remove the obsolete XmlPoke methods.

[patriksvensson]

@devlead We call the obsoleted methods in the generated aliases. I think that is the problem.
Yes, we should remove the XmlPoke methods.

We should also invest in a way of tracking stuff that we obsolete. Like when we obsoleted it (what version).

[devlead]

a warning version and error version options to attribute perhaps.

[patriksvensson]

@devlead Not a bad idea. Let me think some more about this.