Create default values for --nuxeo-action-button-label and --nuxeo-actions-menu-dropdown

Two CSS variables exist to control the padding of the nuxeo-action-menu's dropdown and the action labels: --nuxeo-action-button-label and --nuxeo-actions-menu-dropdown.

More often than not we end up duplicating the same piece of code:

        --nuxeo-action-button-label: {
          padding-right: 8px;
        }
        --nuxeo-actions-menu-dropdown: {
          padding: 0 8px;
        }

Which suggests that we should consider making them the default values, and we can always use these mixins to unset the defaults.