Allow dashboard plugins to advertise what type of objects they support

[mofojed]

As a user, I want plugins to be able to advertise what objects they support opening so that the UI can be smart about display those object types. Should be able to include a friendly name of the object and icon as well (for descriptions).

Currently, if there's an unknown object type exported (such as PartitionedTable), it appears as a button like other objects but just doesn't do anything: #1143

[mofojed]

We could also do some sort of ObjectPlugin that just maps a panel to an object, instead of making a whole DashboardPlugin.

[mofojed]

As we get into programmatic layouts, the plugins we build here should be compatible. To that end, I'm thinking there should actually just be a WidgetPlugin that can advertise it's type, and instead of rendering a whole panel, it just renders the component itself. Panels can then be created/managed programmatically.