Add button to "More Actions" dropdown

fooButton.html.twig

{{ UI.clientButton({
    'dataUrl': path(
        'acme_demo_foo_route', {
            entityClass: oro_class_name(entity, true),
            entityId: entity.id
    }),
    'aCss': 'no-hash',
    'iCss': 'icon-comment-alt',
    'dataId': entity.id,
    'label': 'acme_demo.contact.action.foo'|trans,
    'widget': {
        'type': 'dialog',
        'multiple': false,
        'refresh-widget-alias': 'activity-list-widget',
        'options': {
            'alias': 'foo-dialog',
            'dialogOptions': {
                'title': 'acme_demo.contact.action.foo'|trans,
                'allowMaximize': true,
                'allowMinimize': true
            }
        }
    }
}) }}

fooLink.html.twig

{{ UI.clientLink({
    'dataUrl': path(
        'acme_demo_foo_route', {
            entityClass: oro_class_name(entity, true),
            entityId: entity.id
    }),
    'aCss': 'no-hash',
    'iCss': 'icon-comment-alt',
    'dataId': entity.id,
    'label': 'acme_demo.contact.action.foo'|trans,
    'widget': {
        'type': 'dialog',
        'multiple': false,
        'refresh-widget-alias': 'activity-list-widget',
        'options': {
            'alias': 'foo-dialog',
            'dialogOptions': {
                'title': 'acme_demo.contact.action.foo'|trans,
                'allowMaximize': true,
                'allowMinimize': true
            }
        }
    }
}) }}

placeholders.yml

acme_demo_foo_button:
    template: AcmeDemoBundle:Contact:fooButton.html.twig
    applicable: "@oro_ui.placeholder.filter->isInstanceOf($entity$, %orocrm_contact.entity.class%)"
    acl: [orocrm_contact_view]

acme_demo_foo_link:
    template: AcmeDemoBundle:Contact:fooLink.html.twig
    applicable: "@oro_ui.placeholder.filter->isInstanceOf($entity$, %orocrm_contact.entity.class%)"
    acl: [orocrm_contact_view]

services.yml

services:
    acme_demo.widget_provider.actions:
        parent: oro_ui.widget_provider.action_button.abstract
        arguments:
            - acme_demo_foo_button
            - acme_demo_foo_link
        tags:
            - { name: oro_ui.view_action_provider, group: activity, priority: -100 } # change priority to render button at the right place