Optimized initial toolbar render with toolbar grouping enabled

[mlewand]

Suggested merge commit message (convention)

Other (ui): Improved toolbar rendering time when multiple items are added or removed at once (e.g. during editor initialization). Closes #6194.

Fix (ui): Removing the first hidden toolbar button will no longer throw an exception. Closes #7655.

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Additional information

While working on it I spotted #7655 and decided to fix it while porting old listeners.

[oleq]

Is there any test for this one? Why is it needed anyway?

[mlewand]

In case factory has no matching button then an undefined is returned (from the callback above). It happens if you misspell button name in the configuration.

There's at least one existing test that will fail if it's not handled correctly:

ckeditor5/packages/ckeditor5-ui/tests/toolbar/toolbarview.js

Lines 411 to 426 in e2befc8

	it( 'warns if there is no such component in the factory', () => {
	const items = view.items;
	const consoleWarnStub = sinon.stub( console, 'warn' );
	view.fillFromConfig( [ 'foo', 'bar', 'baz' ], factory );
	expect( items ).to.have.length( 2 );
	expect( items.get( 0 ).name ).to.equal( 'foo' );
	expect( items.get( 1 ).name ).to.equal( 'bar' );
	sinon.assert.calledOnce( consoleWarnStub );
	sinon.assert.calledWithExactly( consoleWarnStub,
	sinon.match( /^toolbarview-item-unavailable/ ),
	{ name: 'baz' }
	);
	} );