Macro method modification policy

[dhardy]

Add a policy on code modification by the #[widget] macro on Widget methods. This code modification was already used (call order verification) but was unobservable outside of reported error cases.

Make use of this policy to call draw.set_id in fn Layout::draw, thus making DrawCx::recurse redundant.

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While allowing macros to modify methods may seem an evil, it is deemed the lesser evil compared to alternatives:

• Requiring repetition of boiler-plate code by users in all implementations. (Note that while the DrawCx::recurse pattern avoided the need to call draw.set_id(self.id()), that pattern is incompatible with a planned future change.)
• Usage of separate "impl-target" and "call-target" methods. This was functional with probe / try_probe but not a great solution, especially when it needs to apply to more than one "method".
• Usage of separate traits for impl- and call-target methods (inevitably this causes name conflicts if the methods share names).
• Usage of a wrapper "pod" as in Druid. This does not pair well with Kas's user-defined widgets.