CoWify style types

[nicoburns]

This:

• Converts parley style types to use std::borrow::Cow instead of references
• Removes the lifetime parameter from PlainEditor and PlainEditorOp by making them using StyleProperty<'static>

This isn't ideal from an API point of view, but does at least allow PlainEditor to be stored within a struct.

It would be an option to leave the lifetime parameter in PlainEditor and make uses who want to store it in a struct specify PlainEditor<'static> themselves. I have not done so because I don't think there's much use for the borrowed editor outside of toy examples.