Shadow DOM: Stop callbacks from inside an open shadow tree

[kleinfreund]

This pull request:

• fixes Stop events occuring in input fields when using a shadow DOM #245.
• supersedes Don't fire events if you're inside a textfield/input in a shadow DOM #254.
• adds tests for open and closed shadow trees.
• adds a sixth options parameter to KeyEvent.simulate. This was required to re-target the event target with closed shadow trees. The related code contains further explanation.

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This is a re-submit of #438 which could not be merged due to breaking tests. That’s no longer the case. No existing tests break. The added tests pass.