File names in search results stay dark gray (and bold)

Since the headers use a bigger font size than the document excerpts, making file names both black and bold, like the matches, makes them too dominant.
Move FFResultNode to its own file
Introduce FFResultsViewController

This moves a lot of view related code out of the Find singleton.
Bump document revision after doing folder search and replace

Without this, the document would not appear as modified (if it wasn’t already).
Allow clearing the (cached) content of a non-open document
Save immediately after folder search‘n’replace on closed document
Remove “Unsaved Changes” warning from find in folder dialog
Use to_s instead of fileSystemRepresentation to guard against nil