Enhance FFAP lookup backend

gf (+lookup/file) has been changed to:

+ Use the active selection instead of the filename-at-point when
  searching for a file (as requested in #4942).
+ Pre-fill the FFAP prompt with the thing at point if no obvious
  filepath can be guessed from it (via ffap-alist).
+ Offer to search the current project for the thing-at-point (if
  counsel-file-jump is available).
+ Fall back to a standard ffap prompt if all else fails.

I've also reversed te order of file lookup backends so that our FFAP
backend is treated as a never-failing last resort (also because the
bug-reference backend is known to fail gracefully, so we can trust it to
run first).

Closes #4942

modules/tools/lookup/config.el

@@ -99,8 +99,8 @@ argument: the identifier at point. See `set-lookup-handlers!' about adding to
 this list.")
 
 (defvar +lookup-file-functions
-  '(+lookup-ffap-backend-fn
-    +lookup-bug-reference-backend-fn)
+  '(+lookup-bug-reference-backend-fn
+    +lookup-ffap-backend-fn)
   "Function for `+lookup/file' to try, before restoring to `find-file-at-point'.
 Stops at the first function to return non-nil or change the current
 window/point.