Import slime-search-property
Fix stickiness problem with REPL text props
Add the ability to perform ops on live terms

Supported thus far:
 * Normalize
 * Show implicits
 * Hide implicits
Improve finding term boundaries
Fix off-by-one error in term replacement
Support active term widgets properly

Now they encode their buffer, making right-clicks reliable. They can
also be added using a menu item.

The active term commands are also documented.
Merge pull request #188 from david-christiansen/active-terms

Active terms