Dummy Plugin -> Empty Plugin

It's clearer what the latter means.
Editable table improvements.

* "Add new row" row is now added on attach using a template, reducing
boilerplate.
* "Add new row" text replaced with a tooltip'd icon button.
* Added a tooltip to the delete button.
* Rows that can't be deleted have their delete buttons disabled rather
than hidden.
Add a margin to plugin cards to allow menu space.

Since elements in core-list are positioned using transforms, only the
last element's margin takes effect, giving enough space below it to
display its menu without scrollbars.