Extend menu plugins api

[juancho0202]

As a distribution developer interested in extending the functionality of the top-app-bar used in OpenSCD I would like to dynamically populate the "actionItems" slot with my own plugins, currently this is hardcoded at the moment of rendering the tag itself with a fixed amount of elements (undo, redo, logs, history, diagnostics).

Acceptance criteria:

1. The "actionItems" slot in the top-app-bar is populated with both the fixed amount of elements mentioned above and any other menu plugin with position "actionItems"

[juancho0202]

@danyill mentions this could be helpful for editor plugins