Discoverable copy paste features

This makes copy/paste a bit more discoverable.

1. Instead of hiding copy/paste buttons, merely disable them. This reduces UI jitter.
2. Add a flash feature to the toolbar plugin, so plugins can flash their buttons if they want to alert the user.
3. Flash feature fires an event, allowing flash-behaviour to be overridden.
4. Copy plugin will flash the paste button when it becomes available.

The corresponding gh-book PRs needed for this to work are here:
  oerpub/github-book#46
  oerpub/github-book#58

This includes and supersedes #79.

It partially resolves http://redmine.oerpub.org/issues/560.