Support rendering multiple ketcher editors on a page without an iframe

[ihuang8]

Background
Add the ability to render multiple ketcher editors on a page without an iframe. The iframe makes it more difficult to access the ketcher instance (e.g. #3693).

Solution

Instead of binding the ketcher instance to the window of an iframe, each instance could be stored in the React state instead.

[AlexeyGirin]

Impossible to verify since it require non-trivial development of Demo app
Will be done in the context of #4145