fix: [#1512] Grab window focus by default for x-origin iframes

[eonarheim]

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

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This PR adds a new engine option (which is on by default) to grab the window focus when hosted in an iframe. This allows keyboard events to work by default.

Confirmed on itch.io https://eonarheim.itch.io/iframe-test

Closes #1512