Add items to Order Detail dropdown menu, instead of the action bar

[martijnvdbrug]

Is your feature request related to a problem? Please describe.
For less urgent actions, it would be nice to be able to add items to the dropdown menu on detail pages, for example the order detail page:

Describe the solution you'd like
I would like to be able to register dropdown menu items, similar to how action bar items are registered:

    addOrderDropDownMenuItem({
        id: 'print-invoice',
        label: 'Print invoice',
        // The route can be a constant value...
        routerLink: ['./extensions/order-invoices'],
    }),

As far as I can see this would only make sense on the Order Detail Page.

Describe alternatives you've considered
Adding to action bar, but with 4+ actions the action bar gets cluttered.

[michaelbromley]

Right now we have the action bar dropdown on only 2 screens:

• product list
• order detail

But I think we should give the possibility of adding dropdown menu items to any list/detail page, just like we do with action bar items.

I'll work on an API similar to addActionBarItem, but named something like addActionBarDropdownItem(). I think this could prove really useful for plugins.