Provide a richer plugin API that handles responses

[stuartmorgan-g]

Currently the fiddly details of constructing an appropriate response object to a platform channel call is left to each plugin. We should change the plugin API to provide functionality that abstracts away those details.

It would also be worth looking at any structural differences between the iOS/Android plugin channel API abstractions and the desktop plugin abstractions; ideally they should look as similar as possible so that writing plugins on desktop doesn't feel significantly different from writing them on mobile.