Hosting mixin: Extend the menu type plugins with the `actionItem`

[trusz]

The CoMPAS fork currently has a compass-session element that wraps the whole OpenSCD and handles authentication, and an action item (toolbar on the top-right) that displays the logged ind username.

We were discussion the following options:

1. A custom addon. We are not there yet to support any kind of addons
2. A new plugin type: actionItem. We wanted to avoid another plugin type
3. A new position actionItem. We decided to go with this solution because it is nicely aligns with the menu plugins and introduces the less amount of changes