Publish collections UI paradigm as API

Dear Edge Insiders, Community, and Program Managers!

I would like to ask you to consider extending extension API with methods allowing developers to implement plugins on the platform used for Collection UI - the pinnable panel is rendered outside on current page DOM.

I think that this API can allow a whole bunch of new useful plugins for MS Edge users.

Frankly, If I think about it, there not that many changes to extensions API, if any. All that Is needed is to give users an option to pin the `default_popup` UI.

What do you think?