Add a method to get configured keystroke.

[jodator]

Taken from: #488 (comment).

After moving keystrokes from UI part of a feature to an Editing part I came to a conclusion that keystroke (ie Ctrl+B - bold as an example) is used in both of them:

1. to define what command to execute in EditingKeystrokeHandler.
2. to show keystroke in Button's tooltip.

Defining the same value in two places is at least prone to refactoring errors in future and will not work after implementing keystrokes config / overriding.

As the keystroke should be defined by Editing part of a feature the UI part should be able to get this keystroke from the editor.

I can see two options:

1. Allow to get keystroke(s) for a commandName (add method EditingKeystrokeHandler.get( commandName ).
2. Attach keystroke to command (as UI already obtains instance of a command).

I think that the first is better in terms of current API as it does not require to have those commands to be executed in any particular order:

// In BoldEditing:
editor.commands.add( BOLD, new AttributeCommand( editor, 'bold' ) );
editor.keystrokes.set( 'Ctrl+B', 'bold' );

// In BoldUI:
const keystroke = editor.keystrokes.get( 'bold' );
view.set( {
	label: t( 'Bold' ),
	icon: boldIcon,
	keystroke,
	tooltip: true
} );

[jodator]

Also: #488 (comment).

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