Refactors the Windows embedding internals to make an engine object that
owns things associated with the engine rather than the view, and updates
the API surface to allow using the engine directly.

This is an incremental step toward both a cleaner, non-struct-based
internal structure and a finalized API surface.

* Implement Clipboard hasStrings method on iOS

* Test call to hasStrings

* Formatting fixes

* Move test to its own file

* Alphabetical order

* Update licenses

* arguments nil instead of empty dictionary

* Guarantee hasStrings will be true when tested

* Formatting