Embed activity fix 2

This pull request is based on #6515.

It adds some commits that adds new functionalities in order to get CSS asynchronously, fixing #6517.

@ewinslow preferred to have 'css!' plugin outside Elgg API, otherwise I think it is very important to give support to libraries to include their own `css`, as in the case in 7d44a91 with colorbox.

Thoughts?

- [ ] resubmit to 1.x branch