Add shortcuts and other fixes, start changelog

[mirisuzanne]

• 💥 BREAKING: Updated keyboard shortcuts
  to match PowerPoint,
  including command-. as 'end presentation'
  rather than 'toggle full-screen' (now command-shift-f)
• 🚀 NEW: Support for blank-screen shortcuts
  (inspired by Curtis Wilcox)
• 🚀 NEW: Control panel includes toggle for keyboard controls
• 🚀 NEW: Control panel buttons have aria-pressed styles
• 🚀 NEW: All slide-event buttons that toggle a boolean state
  get aria-pressed values that update with the state
• 🐞 FIXED: Scroll to the active slide when changing views
• 🐞 FIXED: Control panel view toggles were broken
• 🐞 FIXED: Control panel prevents propagation of keyboard shortcuts
• 👀 INTERNAL: The current slide is stored in an activeSlide property

Related Issue(s)

Reminder to add related issue(s), if available.

• fixes Make sure that keyboard shortcuts match common apps/remotes #3

Steps to test/reproduce

Please explain how to best reproduce the issue and/or test the changes locally (including the pages/URLs/views/states to review).

• try the various shortcuts
• try the buttons in the control panel

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