Fix plugin selectors after angular upgrade

The recent angular update changed the format of the hash keys.
They're now in the format 'object:123' (etc), which isn't a valid
CSS class name any more, but we used the $$hashKey as such.

I used this opportunity to introduce a bit of abstraction there as well
Merge pull request #466 from glowing-bear/fix-plugins

Fix plugin selectors after angular upgrade