OnMessage trait

[dhardy]

There are a couple of challenges with the existing per-child-widget message handler system:

• binding the same handler to multiple children is tedious
• binding handlers to generic parent widgets such as List cannot work in the same way

Instead, this attempts to bind handlers based on the message type by requiring parent widgets implement a new OnMessage trait. But first we need a couple of auto-implementations:

• children with VoidMsg message type should be supported automatically
• children with the same message type as the parent should automatically forward the message

Problem: these generic implementations overlap. Except: we already worked around this via derive(VoidMsg): each viable message type supports From<VoidMsg>, thus we can implement a rule for any M: From<N> where M and N are the parent's and child's message types respectively.

Problem: implementing a specific handler which is not covered by the above rule is now reported as a conflict: rust-lang/rust#90587. This blocks further progress here.