A loading state for plugins

using [paper-progress](https://www.webcomponents.org/element/PolymerElements/paper-progress)

that shows a loader at the top of the plugin
and can be enabled/disabled from the plugins

Feedback? 👍 or 👎

Start by locating how plugins render and expose their enabled or disabled state, then review the paper-progress component linked in the issue. The work is done when a loading indicator appears at the top of a plugin and can be enabled or disabled through the plugin configuration.

Written by the indexing model from the issue text.