Fix layering of sc() keybindings with vk() ones

[lhecker]

The quake mode keybinding is bound to a scancode. This made it
impossible to override it with a vkey-based one like "win+`".
This commit fixes the issue by making sure that a KeyChord always has a vkey,
and leveraging this fact inside ActionMap, which now ignores the scan-code.

PR Checklist

• Closes [1.11 blocking] Overloading win+` doesn't work anymore #10875
• I work here
• Tests added/passed

Validation Steps Performed

• quake mode and other keybinding still work ✔️
• Repro settings from [1.11 blocking] Overloading win+` doesn't work anymore #10875 work correctly ✔️

[lhecker]

I've rebased this PR to resolve merge conflicts with #10907, added a comment and removed redundant code from IslandWindow::RegisterHotKey.
The diff is a bit more compact now.

[DHowett]

@msftbot merge this in 18 minutes

[ghost]

Hello @DHowett!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I won't merge this pull request until after the UTC date Wed, 11 Aug 2021 18:13:56 GMT, which is in 18 minutes

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".

[DHowett]

(build's failing though so it will ignore me 😄)

[ghost]

🎉Windows Terminal Preview v1.11.2421.0 has been released which incorporates this pull request.:tada:

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