Rename Response

[dhardy]

The enum Response { Used, Unused } is a left-over from before #309, where event handlers returned a message directly. Now the type is used only to determine whether an event was used/consumed. The name doesn't fit current usage and overall the type feels clunky.

Actual usage is:

• To report whether Widget::handle_event consumes the event, and thus whether to call handle_unused on ancestors
• To report whether steal_event intercepts the event

Suggestions:

• Remove, instead returning bool. Problem: less clear exactly what true/false means. Problem: potential for returning result of a function call incorrectly.
• Re-export Used, Unused directly, including in the prelude. More namespace pollution. Still need the type name in the method signature.
• Rename, e.g. to IsUsed. Probably the best option, but the name is uninspired.
• Replace with pub struct IsUsed(pub bool)

[dhardy]

Another suggestion: Consumed.

But I'm not convinced this is an improvement over Response. It can stay as is for now.

[dhardy]

Done: #406