Make layout_t::update_metrics public
Add support for printing

Closes #517.
Let a document search clear the folder results used by ⌘G

If the user does a folder search then ⌘G will step through all the matches, opening documents as required. Previously the user would have to do a new search, using a different search string, to stop ⌘G from moving to a new document. It is now possible to stop the behavior using ⌘F followed by ↩.