Add an "insert snippet" button

[ejucovy]

When editing a checked-out code block there should be a button to "insert snippet". When clicked, this would list all currently available snippets, and let the user pick which one to include.

It would then just write the <include /> tag into the document at the cursor. (So, this will need some at-least-light integration with Ace.)

It would be even nicer if there was a way to interact with this button using only the keyboard.