KeyboardShortcuts: Use a separate Mousetrap instance per component instance

[jblz]

Fixes the same issue as #2085, but allows for multiple bindings per key command (props @aduth for the alternate approach).

Without this change, the callbacks are not being properly unbound when the visual editor unmounts.

To Test

Confirm Existing Behavior

• Run release or master
• Create a new post via: /wp-admin/admin.php?page=gutenberg
• Add some content
• Switch to text mode
• Issue a keyboard command...for example: cmd+a for "select all"
  NOTE: You may need to click the editor whitespace again to be able to kick this off
• Confirm the Visual Editor's select all callback is executed:

Confirm the fix

• Run this branch
• Repeat the above
  • Callbacks should only be executed in the visual editor mode (none are currently registered via this component in text mode)
• Attempt to bind the same key command(s) as are already bound via a separately instantiated KeyboardCommands compoent
  • All bound callbacks should execute and unbind when the component unmounts.

[codecov]

Codecov Report

Merging #2169 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #2169   +/-   ##
=======================================
  Coverage   22.23%   22.23%           
=======================================
  Files         136      136           
  Lines        4273     4273           
  Branches      721      720    -1     
=======================================
  Hits          950      950           
- Misses       2806     2807    +1     
+ Partials      517      516    -1

Impacted Files	Coverage Δ
components/keyboard-shortcuts/index.js	0% <0%> (ø)	⬆️

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[aduth]

I don't know what's going on with the codecov failure, but this looks good to me 👍

[aduth]

Aside: I think this will make for an easier path toward context-scoped keyboard bindings, as mentioned in "Implementation notes" of #1944