Create MenuSection Component

In order to create consistent action menus, we need a few new UI building blocks.

The plan is to make a <MenuSection> component which will serve as a wrapper for various UI elements. It will accept a label as a prop.

The rest of the menu should be composable by existing components such as Checkbox, RadioButton, NavList/NavListItem etc.

For regular action menus, we need to update the dropdownItem button-style as well.

The end result should be something similar to this:

These components will be used to provide a custom action menu for PaneHeaders (https://issues.folio.org/browse/STCOM-388) but will also be useful in other aspects of the UI.