Add composition action for supporting IMEs

[jgraham]

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WebDriver key actions don't cover the case of IME inputs, in which multiple composition events end up inserting one or more characters that don't directly map to the keys pressed. This represents a problem for testing web editors which need to handle IME related events specifically.

The relevant web specification here is https://w3c.github.io/uievents/#events-compositionevents

It makes sense to model this input as actions, because it represents a series of input events that can be interleaved with other input events (e.g. mouse clicks).

Gecko has a couple of test APIs which demonstrate the detailed functional requirements; obviously the details need to change to fit into the WebDriver model, but I propose using those as a guide to what we need to support to be useful for testing web editors.