Add Toolbar component

[pfferrari]

What I did

Added a new Toolbar component that generates a horizontal navigation menu made of Buttons and Dropdowns configured by its props.
Its menu items are configurable by filling the items prop and they are rendered as Buttons as default behavior.
Each menu item could have a grouped hierarchy of dropdownItems to define the sub menu shown by clicking it.
If set, the dropdownItems prop will cause the current item to be rendered as a Dropdown with a Button as clickable label.
Thanks to the grouped behavior of the dropdownItems prop a divider line will be automatically rendered between each group.

Example usage

<Toolbar
  items={[
    {
      label: 'Primary',
      size: 'small',
      onClick: () => {
        console.log('Primary')
      }
    },
    {
      label: 'Secondary',
      icon: 'pulse',
      variant: 'secondary',
      size: 'small',
      onClick: () => {
        console.log('Secondary')
      }
    },
    {
      icon: 'dotsThree',
      size: 'small',
      variant: 'secondary',
      dropdownItems: [
        [
          {
            label: 'Edit',
            onClick: () => {
              console.log('Edit')
            }
          },
          {
            label: 'Set metadata',
            onClick: () => {
              console.log('Set metadata')
            }
          }
        ],
        [
          {
            label: 'Delete',
            onClick: () => {
              console.log('Delete')
            }
          }
        ]
      ]
    }
  ]}
/>

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests).
• Make sure to add/update documentation regarding your changes.
• You are NOT deprecating/removing a feature.

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